ELI5 Why is the chemical GenX considered safe in Telflon pans despite having the similar carcinogenic implications as it’s precursor?

Read the Story

Show Top Comments

Well, tldr: because it is safe, in the form of Teflon (which is a solid polymer), but as a monomer, it is dangerous to humans.

By itself, it is dangerous, but that hasn’t stopped the use of dangerous chemicals, just increased safety standards during the manufacture.

Once you change the chemical bonds inside the molecule, the properties are changed. Chemistry works in a weird ways, for example, the same molecule can be either deadly or an effective drug to combat severe desease (enantiomers).

Now, to explain why it is still used and how it is safe (kinda) think about table salt and gold production.

Table salt (NaCl) is made of sodium (explosive when exposed to oxygen) and chlorine (deadly as pure gas). Together they make a lattice (cage-like structure) through bonds (almost polymer like), and salt is produced, which is safe to consume.

Polyethylene is made of ethylene, which is toxic. And the same story goes for plastic bottles made from PFTE that carry water, milk, soft drinks, etc.

Gold uses cyanide heap leaching. Cyanide is a very potent poison. Release of gaseous cyanide in a factory guarantees almost no survivors. BUT the safety precautions associated with this process are very high. Therefore it is still used today. And many produces use dangerous chemicals as a starting material.

So at the end of the day, Teflon is safe to use, and it’s production is controlled to ensure noone gets harmed (obviously depends on the country safety standards).

Edit: before people say that Teflon is still carcinogenic, yes it might be, if you eat it in large amounts over years. Smoking (and even bacon, according to some sources) are carcinogens, but that doesn’t stop people from consuming such. And if we get down the line of conspiracy theories, then well, I don’t know what else to say.


Oh they were DEFINITELY carcinogenic and caused birth defects. Watch *The Devil We Know* documentary for the full history and how that all went down.

Basically, after years and years of lawsuits and back and forth they finally got a specific chemical marked as carcinogenic and Dupont voluntarily stopped using it…. and replaced it with this chemical. This means that everyone has to start from scratch to prove that THIS chemical does basically the same thing. It’ll take another twenty years. The fact that it’s chemically similar doesn’t mean it starts in a different spot, legally, really.

And I haven’t used a nonstick pan since I watched that documentary. Powerful stuff.


There is this journalism professor who is really into cooking called Adam Ragusea. He made a great video essay on YouTube in this that you’d find helpful. The man does extensive scientific research and is trustworthy.


It’s precursor of what? Please explain.


Because chemical companies race against research to utilize toxic substances until they get caught.


Eli5: How is it that Cows can consume the same fibrous grass that makes every other herbivores scat into tight balls, somehow manage to produce turds of such low density as to be called “cow patties”? ?

Read the Story

Show Top Comments

Basically because cows are not as good at conserving water as other herbivores. Because their bodies aren’t trying to save water they don’t spend the energy to reabsorb the water in their poop. An herbivore who lives in an environment where water is scarce wants to loose the least amount of water through its poop as possible, so their bodies spend the energy to pull as much water from their poop as possible. Water is not a limiting resource for cows, so their bodies are not adapted to be efficient at saving water.


Cows have a digestive system heavily optimized for grazing on low-nutritional-value grass.

Your small intestine is 5m, theirs is 40m. Your stomach is a small bag, theirs is a giant 4-chambered complex organ that they use like a fermentation vat.

Plant matter that passes through this system is thoroughly destroyed and fermented by bacteria into nutrition for the cow.

“Non-ruminant” herbivores that don’t use this layout tend to leave more undigested fiber in their poo.


can someone explain the QUESTION like im 5?


This is a little different than OP, and doesn’t need to be ELI5, but can someone explain why cows poop differ from sheep/goats when all are ruminants then?

They’re going to be different than hindgut fermenters which is kinda the root of OPs question, but goat scat versus a cow’s are very different even though they’re in the same family (Bovidae)


Because they basically have constant diarrhea. The bad thing about diarrhea is that you get dehydrated quickly so in the wild, they had to put more energy in conserving water. But domesticated, they were taken care of, so they had an abundant supply of water, and breeding (and thus evolution) could focus on other areas.


ELI5: Why did cyan and magenta replace blue and red as the standard primaries in color pigments? What exactly makes CMY(K) superior to the RYB model? And why did yellow stay the same when the other two were updated?

Read the Story

Show Top Comments

Red Yellow Blue was used in early printing because that was the best they knew at the time. Technology advanced, and printing color images switched to using magenta and cyan to get a wider, more accurate range of colors.

Our eyes see three basic colors of light: Red, Green, and Blue. Cyan pigment only absorbs one color from light, Red; this leaves behind Blue and Green. Magenta pigment absorbs only Green. Yellow pigment absorbs Blue. Mix those three in varying combinations and you can get every color the human eye can perceive. (Technically you don’t need Black in CMYK, but it takes a lot of CMY to get a good, dark Black.)

In contrast, Red Yellow and Blue can only be mixed to *most* visible colors, but there are some that are simply not possible to create with those three as your “primary” colors.

The reason Yellow “gets a pass” is because it is a primary color in color schemes for both subtractive light and traditional art.

So why did we use Red Yellow Blue for so long? Because (a) for a long time we didn’t know enough about light to know Cyan and Magenta are better primaries, and (b) Cyan and Magenta pigments are hard to make, requiring (relatively) modern processes.


CMYK does not account for the color white. It does not need to because it is designed to be used for color renditions to be printed on white paper; white just means “no ink”. With CMYK, the color produced by combining all colors to full saturation is a dark brown. With printed inks, greater application of ink reduces light reflected and results in a darker image. This is noteworthy because it’s the opposite with light renditions. On to RGB!

RGB, on the other hand, does not account for the color black. It does not need to because it is designed to be used for color renditions to be produced with light; black just means “no light”. With RGB, the color produced by combining all colors of light is white. This is why RGB is used for computer screens. It can render white. CMYK cannot. For PC display hardware, black backgrounds do not reflect light emanating from nearby lit pixels, so black is an ideal color for the screen’s backing. It can display black by simply not lighting a pixel, and the pixel will hold black well enough because it reflects minimal light from the black backing. If the backing were right (so an off pixel displayed white), the white would reflect colors of nearby lit pixels and not appear white at all.

Thus, CMYK for printing and RGB for light renditions that require production of the color white.


>**EDIT:** to clarify my questions a bit, I’m not asking about the difference between additive/subtractive color models which has already been covered in other threads on this sub. I’m asking why/how the older Red-Yellow-Blue model in art/printing was updated to Cyan-Magenta-Yellow, which is the current standard

Oh but you are. It has -everything- to do with subtractive color models.

Blue, Yellow, Green, and Red inks were among the first to come out, so they got used in microdotting early on. The big problem with these four colors is that you can’t make most of the colors (because of subtractive color that pigmentation works on); this made things look grainy, lacking color depth. It’s one reason why comics before a certain time look like crap.

Once cheaper dyes and computers became ubiquitous in publishing, they developed the CYMK coding model so that computer-information could easily be shifted into color printing. CYMK doesn’t have much translating needed to convert from RGB (additive color, which is how graphics is stored and processed in most computer use), and so that also became attractive as What You See Is What You Get (WYSIWYG) publishing became the norm.

The reason black is added as a fourth color is simply because of cost: Black ink is FAR cheaper, and blacker, than if you made black out of the three color inks.

As for what was previously used, there were various color models, each with advantages and disadvantages.

Four-color used Red, Blue, Yellow, Green, and Black. It did not mix Yellow and Blue to get Green ink because that would be expensive–it was cheaper just to use Green ink. Because it used those four primary colors, this was called ‘Four-color printing’ and was the norm until inexpensive Cyan and Magenta inks became available and computers were the norm.

Another form of color printing used was ‘Spot Color’ where two or three colors of ink were used. This could be used to get exactly the color you wanted, but you’d only get that color, and usually the second color was black. This is considerably less expensive than Four-color, but it’s not the least expensive option.

After this, you have good’ol Monochrome. Usually, black, but sometimes a different color, that’s when they use one ink and one ink only.

Unless you know what you’re looking for, you might think a given book is full color throughout, but it actually might not be. It might be using a combination of four CYMK pages interleaved with spot-color pages to give the illusion of full color, while costing a lot less to publish. This might explain why your favorite gaming book doesn’t have the table you want on the page you’d think it should be (because it uses a different spot-color than the page you’d think it be on) or why it seems some sections of that book have full color art, and then long sections with only text, but those text pages have that metallic gold lettering you ever see on the full color pages (CYMK doesn’t have metallic sheen or similar things)


>Why did cyan and magenta replace blue and red as the standard primaries in color pigments?

Painters used “true” blue and red because 1) the pigments are easier to obtain in nature 2) people were still learning how color works 3) R and B were good enough to recreate the majority of colors around them. The range of missing colors (like magenta, bright purples, oranges etc) arent often needed in paintings of natural subjects.

>What exactly makes CMY(K) superior to the RYB model?

It’s “superior” because you can create more colors with that system. As others have said- you can mix M with Y and C to make R and B and many other colors you can’t get with RYB. RYB makes “muddier” colors. I put superior in quotes because many painters still use an RYB palette, because it’s easier to make those duller, earthier colors. Modern printers need to make ALL the colors though.

>And why did yellow stay the same when the other two were updated?

They just had yellow correct from the beginning. Again- they work as primaries for many many colors. Just needed a bit of refining to get more.


RYB has a smaller gamut than CMY, but it might be easier for painters using pigments that are more opaque and difficult to mix. Dyes being mixed by computers are much more precise, far beyond what a painter can handle on their palette.

Ultimately it is just a less capable model which has a lot of institutional inertia behind it.


ELI5: how can facial skin be dry and oily at the same time?

Read the Story

Show Top Comments

If you over wash your face or use a cleaner that is a stringent, your skin becomes overly dry and your body over produces oils to compensate.

I used to have a pretty wicked case of the issue you are describing.

I found that a sensitive face wash without any type of salicylic acid to be best (burt’s bees foaming cleanser works great for me)


Water. The oil is there to prevent the evaporation of water (drying) of skin. If you wash away all the oil and water in/on your skin, it will be dry. Your body will re-cover the area with oils, but oil doesn’t replenish the water (just protects from evap).

Use moisturizer immediately after washing and stay hydrated to combat dry skin.

Personal opinion: you don’t have to use intense soaps/scrubs every day unless you have a specific medical condition you’re dealing with. Washing face with your usual body wash (usually much less harsh than soaps or deep cleansing face washes) is enough for daily routine.


You can be dehydrated and have naturally oily skin.

You can be dehydrated and have naturally dry skin.

It’s an important distinction to figure out when caring for your skin.


Oil is not water. Oils allow the skin to retain moisture better. If you wash your skin too often with detergents, you are washing away the oil and water can evaporate easier. This tells your body to produce more oil and too much oil is not good for the skin either. If you leave soap on your skin for too long, you will get chemical burns. This tells a bit what the soap does. Water alone does no damage. The liquid soaps we have are concentrated and the amount used is much less than any advertisement or TV show or a movie shows us. The actual amount needed is often ten times less. This applies to washing up almost anything, hair, body, dishes… More elbow grease and less detergent, plenty of water.

If one washes too much, don’t quit cold turkey. Take time to give your body and skin to get used to the new regime. Your skin has loads of bacteria, no matter how much you wash. Healthy skin also has good bacteria covering it and it takes time to get that biome working for you and not against you. Washing too often with soap leaves behind bacteria strains that can take soap for longer time and can live from the excess oil being produces in between. In fact, if you have very, very steady routine they can evolve to form spores before you wash up, the next generation will survive the “purge”. You are training them to be as effective as possible while the more benevolent and useful bacteria strains are constantly killed. If you stop right away, the “bad” bacteria colonies will explode in size.


Dry = lack of oil
Dehydrated = lack of water

You can have oily skin that is dehydrated because water is evaporating out of it (transepidermal water loss), which makes it feel “dry” because it’s dehydrated (needs water) even while producing oil.


ELI5: Why can it be difficult to stay awake while driving, but when pulled for a nap it takes effort to fall asleep

Read the Story

Show Top Comments

For most of us, driving is a mundane task that we can do on autopilot. As a result, your mind isn’t focused and the tiredness can prevail. When stopping to take a nap, you’re now focusing on something (trying to nap) and this keeps your mind active. Similarly, you may have sleep anxiety because you know you need a nap.

This applies to other things like falling asleep while watching TV then being wide awake once in your bed.


I forget the specifics, but the main reason seems to be micro-vibrations in your ears/cranial nerves cause a relaxed-perhaps hypnotic-state. This is seemingly why the old “take the baby for a drive” trick works so well… not because car rides are a mundane activity for babies.


In the book mans search for meaning – author talks about logotherpy and parts of it have been proven. Specifically, when you actually want to have a certain outcome, it’s sometimes more effective to try to make the opposite happen.

For people with sleep anxiety and issues falling asleep, it’s easier to fall asleep if you try to stay awake while in a comfortable spot. I’m not sure the science of it but when I’m sitting in a meeting/driving/sitting at my desk at work and I’m trying my best NOT to sleep, I get so fucking tired it’s unreal. Then when I’m in my comfortable bed wanting to sleep my mind races. Could be this opposite effect thing working it’s magic.


The road plus the stuff going past you and the constant humming of the engine can be very hypnotic, coupled with the fact that you tend to be bored often.. simmilarily train crews struggle with this also it’s a very monotonous job staring at the rails watching them go past puts a guy to sleep fast we often have to get up and walk around the cab or talk to each other constantly to stop from sleeping. You wouldent think something loud like a train would do this but let me tell you that engine is the most relaxing thing ever for some reason but only when you dont want it to be


Think how great it will be when we can just push the “autonomous drive” button and nod off for a bit on the highway.


ELI5: Why do vocal harmonies of older songs sound have that rich, “airy” quality that doesn’t seem to appear in modern music? (Crosby Stills and Nash, Simon and Garfunkel, et Al)

Read the Story

Show Top Comments

I see a lot of good info, but I didnt see anyone talk about this. When people sing in the same room the vibrations of their voices actually affect each other. When perfect harmonies are sung there are natural overtones created by the stacking of the sound waves. When voices are autotuned or electronically harmonized you are actually missing a lot of frequencies that natural harmonization would have, making the newer stuff sound flat and robotic.


Check out a documentary called The Harmony Game. In it, the producer of many Simon and Garfunkel classics details his vocal recording and mixing style which basically amounts to having each vocal recorded and doubled individually and then both vocals on one mic giving the mixer several tracks to pan and balance.


They loved doubling up vocals back then, as in you have the same vocal track repeated a fraction of a second later. John Lennon is doubled up on nearly all of his songs. They’d do this with harmonies as well – each vocal onto a single track and then doubled up. That’s a lot of vocal going on at once, with sounds overlapping and interfering with each other, giving it that swirling shimmery sound.

What I also notice about the example you posted is that every vocal harmony is at a similar level, as if you’re listening to a group of singers in a room. Modern music tends to go with the lead vocalist pushed to front, and backing singers for the harmonies, pushed further back in the mix.

Any kind of commercial music is competing in a kind of arms race of sound, attempting to stand out. Producers come up with a trick that makes their song sound bigger, then pretty soon everyone’s doing it. Vocal doubling was one of those tricks. As we move into the 80s, the backing track becomes more of a focus. There’s only so much you can do with vocals, but instruments and production techniques are changing all the time.


Part of it might have to do with the loudness war and songs being mastered for shitty audio gear now days. The loudness war is artist wanting their music to be mastered louder and louder, which results in less fidelity in the song because its all kinda jammed up there rather than using the full spectrum.


Overdubbing was common even in this era; in fact, the effect here is produced by multitracking, a form of overdubbing in which multiple takes of the same part by the same vocalist(s) are overlaid and bounced to a new track. What you were told about acoustic disturbance isn’t totally inaccurate, as the perceived effect itself comes from the constructive & destructive interference of the overlaid waveforms, but this doesn’t require that they interact in the air, which you can test for yourself just by graphing any two simple waveforms and then graphing their sum. The same waveform summed with itself will produce the same waveform but with twice the amplitude/volume (1+1=2 (constructive)), while a waveform summed with its inverse will produce silence (-1+1=0 (destructive)). Where no two takes of a part will ever be identical, the multiple waveforms interact with one another in such a way as to create a complex pattern of interference, reinforcing & attenuating certain frequencies in a non-fixed way, which we perceive as this ‘airy’ quality you describe, and which cannot be produced quite the same just by processing the signal with a unison or chorus effect as is common today.


ELI5: How did the 3D effect on Nintendo 3DS work without glasses?

Read the Story

Show Top Comments

For those who are unaware, there are even phones that have 3D cameras and 3D screens. I don’t know of any new phones that do this, but six or seven years ago I had a phone called an Evo 3D. the 3-D effect work very well with the camera and the pictures look very good on the screen.


Also fun to know, the heads-up display of certain cars uses that effect. It’s very neat to see that small display floating above the car’s nose.


From what I’ve read now, The Nintendo 3DS’ top screen utilizes a filter called a “parallax barrier.” One of the images necessary for seeing 3D is projected to the right and the other image to the left. The images occupy alternating vertical columns of pixels and are filtered through the parallax barrier.

The barrier acts as a vent to project the images and ensure they hit your eyes at the necessary angles to produce the desired 3D effect.


ELI5. How do you bring back a species from from near extinction?

Read the Story

Show Top Comments

Serious inbreeding can be avoided with 200 animals. That’s enough to introduce enough genetic mixup without causing too much of a problem (if there were only 2 rhinos, and the whole population had to stem from that, you’d have potential problems).

There is something called the 50/500 rule made in 1980 by Australian geneticist Ian Franklin and American biologist Michael Soule. This rule suggests a minimum population of 50 in order to combat inbreeding, though a minimum of 500 to reduce genetic drift.

This is a bit of a generalization, and does vary species to species (species who have higher “litters” like mice and insects can tolerate lower populations than species with lower numbers (like large mammals or huge trees). So in the case of a rhino, 200 is probably enough to introduce enough genetic variability to avoid major inbreeding problems, but there may be some genetic drift, and if the population were to balloon back to normal wild levels, those Rhinos may be significantly genetically different from the wild Rhinos of old as a result of so many offspring coming from a small source gene pool.

EDIT: Wow, thanks for the silvers, responses, and support! I’m happy to have helped, in some small way, make the internet slightly more useful than a big porn box.

EDIT2: Now GOLD?! You flatter me, Reddit!


I remember reading about how all giant squids, regardless of which ocean you find them in, have incredibly similar DNA. This could be a result of giant squids becoming nearly extinct at some point in time, and then later their population is restored, but it came from a limited gene pool, that is, the few survivors.


If i remember the paper correctly, you can re-breed from as few as ~30, though at that point it requires VERY carefully management and there will be some….unfortunate ones. Anything in the 3 figures is pretty manageable assuming the animals will breed in at least relative captivity, which isn’t guaranteed.


A very rigorous and extensive planned breeding program. Like others said, as long as they aren’t too genetically similar to start with, 200 is easily a big enough genetic pool to produce offspring without inbreeding effects. You just have to ensure, through relocation or captive breeding, that the various populations intermingle and continue diversifying. So you start by cataloging the existing animals and their genealogies as far as you can. Then you closely monitor the existing animals and ensure different lines are interbreeding. If a population becomes too isolated and begins trying to inbreed, then you move some viable adults from a different population into the stagnating one. It’s a lot of work.


Not only can inbreeding be avoided, but inbreeding isn’t a genetic death sentence. It’s kind of a secret weapon for species coming back. Unless they get really unlucky, the carriers of defective genes die off without breeding, so after a few generations, the species has relatively broken free of any genetic defects caused by inbreeding.

They lack genetic diversity, and are more likely to have genetic diseases, but they still live on


ELI5. Why are large passenger/cargo aircraft designed with up swept low mounted wings and large military cargo planes designed with down swept high mounted wings? I tried to research this myself but there was alot of science words… Dihedral, anhedral, occilations, the dihedral effect.

Read the Story

Show Top Comments

Military cargo aircraft use high mounted wings because it allows them to use unprepared or hastily prepared runways. Keeping the engines up high helps with not sucking in a bunch of dirt and rocks. Passenger aircraft operate pretty much exclusively from well maintain airports, so that isn’t a big deal for them.

Upswept wings make a plane more stable in a roll. The aerodynamics work out so the plane’s natural tendency is to want to roll back to wings-level. This makes the plane easier to fly, and generally more comfortable, but limits the rate at which it can roll.

High-wing large transports usually already have quite a lot of roll stability, so downswept wings are used to give them slightly more responsive handling, which helps when landing in adverse conditions.


Military cargo planes are desired to be very close to the ground for easy loading and unloading of extremely heavy cargo. So the whole plane is reconfigured to avoid banging the wings and engines into the ground.

also they are used sometimes on bad quality runways which may contain dirt and gravel, so again there is a desire to pull the engines up away from debris.


Military transports have a high-mounted wing in order to get the bottom of the fuselage as close to the ground as possible, so you can drive vehicles into them via a built-in ramp. It also reduces the obstacle clearance requirements on crudely-built forward-area runways.

The higher the wing is on the fuselage, the more stable the aircraft is in the yaw and roll axes. Airliners have dihedral (upswept wings) to take advantage of this. Military transports, with their high-mounted wings, would be *too* stable with dihedral — so they have anhedral (downswept wings) to offset it.

There is one airliner with high, anhedral wings, the BAe146. Many of its passengers can’t see the scenery because the engines are in the way — worse, its only emergency exits are at the ends, because if you tried to abandon it amidships you’d run into a hot engine.


Airliners prefer to have low mounted wings and low mounted engines because lower engines are much easier to reach. In fact, a big selling point is often that the low engines don’t need much complex equipment to reach. Just an elevated platform and you can basically strip the thing down if you have to.

Low mounted wings are also much easier to land as the ground effect is much more pronounced, but a disadvantage is not being able to have a lot of clearance between the wings and the ground on the ground. So you can’t have lots of people darting around under the plane the same way you could with a military cargo plane.


Speaking of cargo, cargo is a huge factor that goes into how you build a plane. Every plane wants to carry as much cargo and as efficiently as possible. For commercial planes like the 747, they are a mix of carrying passengers in the crew compartments and luggage, mail, or other goods in cargo areas.


For a military transport, you basically have to carry extremes, either a huge amount of passengers like paratroopers or no passengers and only tanks or vehicles, so 1 giant cargo hold is better than having the plane cut in half for specific loads.


Also you want to be able to access said cargo. You could use a lift like a commercial plane, but having high mounted wings means the fuselage can be MUCH closer to the ground. So you can literally just drive off the plane. For a 747 or A380, you could carry vehicles in it, but you would almost certainly need a crane to get it out, a C-150 could just open up and you could drive the car off.


About “dihedral” and “anhedral”:

Those words just refer to the wing design shape you’re talking about. Wings bent down is anhedral and wings bent up is dihedral. Dihedral is good because it improves the plane’s stability while flying while anhedral makes it worse. We like our civilian passenger planes nice and safe and stable so we design them with dihedral. Military planes, like the c-5 and c-17, use anhedral not because they are made to be unstable, but actually because the wings create so much lift to make such a heavily loaded plane fly that they actually bend upward and have a slight dihedral while in flight


ELI5: Why isn’t the night sky just one big light?

Read the Story

Show Top Comments

Two things:

1) Regardless of how big the universe is, light still takes time to get places. The universe is 13.77 billion years old, so light has only had 13.77 billion years to get here. Because the universe is expanding, we can see stuff from much farther away than that, but there’s still a limit on how far away stars can be and still have had time for the light to get to us.

2) As the universe expands, it stretches light passing through it, causing the light to be redshifted, which means it lowers in wavelength. Visible light from the very edges of the visible universe can get redshifted out of the visible spectrum and into infrared or radio waves. That’s why the Cosmic Microwave Background is, well, microwaves. It used to include a *lot* of visible light, but it’s so old and it’s been shifted so much that it’s all microwaves, now.


it’s called olbers paradox and actually there is a lot of light. we just can’t see it because it’s out of out visible spectrum. this is because as galaxies move away the light changes and so we may not be able to see it anymore


Lots of nice comments, and several explain the physics in a way I’ve forgotten since I studied it, so kudos.

But I’d like to add that the night sky is really actually quite bright. If you can get somewhere without massive light pollution, there really isn’t any direction which doesn’t have light.

If you find a “dark patch” and look at it through a telescope you’ll generally see stuff… And if there’s a dark patch in that then you get a bigger telescope etc.

Edit: cause apparently I can’t English today


1) Light diffuses rather significantly with distance. The sky is indeed awash with stars, but most of them are too far away to be even remotely visible.

2) Because of universal expansion, light from far away sources gets redshifted to frequencies below that of human vision limits.

3) Expanding on the above; the night sky, bluntly, **is** one big light. But most of that light is at relatively low frequencies below what the human eye can see, even before redshifting is taken into account.


If you draw a line on a balloon with a sharpie, then inflate the balloon, the line you drew will get stretched. What was one solid black stroke at the beginning is now a large faded line.

Now imagine the balloon is the universe and the line is light from a far away object. Eventually it gets stretched so much that we can’t see it anymore.