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Hank: It’s Thanksgiving here in the United States and we at SciShow are thankful for many things, but one of the big ones is the fun and curious questions we get from you! In this holiday compilation video, we’ve decided to organize it like a meal.

Which means we get to start with an amazing appetizer and one of the best questions we’ve ever been asked: If I weigh 99 pounds, and I eat a pound of nachos, Am I 1% Nacho?

Hello and welcome to SciShow Quick Question, where we answer the questions the Internet asks. The following was asked on my Tumblr a while back: “If I weigh 99 pounds and I eat a pound of nachos... am I 1% nacho?” Sounds like not a very complicated question, but it turns out to be, like, super complicated, because buried in this little question are a number of bigger questions.

First, what are nachos. We can safely say that this [SHOWS NACHOS] is a plate of nachos. Invented, by the way, in Mexico for Americans... it’s not a traditional Mexican dish. If I chew up the nachos, though, are they still nachos? If my digestive system dissolves them into a mushy, acidic goo... are they still nachos. I would venture to say, well, if you took what was left in my stomach an hour after I ate nachos and put it on a plate, zero people out of seven billion would call that “nachos.” HOWEVER for the sake of argument, let’s say that the atoms and molecules that comprise the nachos are still nachos after you masticate and begin to digest them.

In that case, the other question is... what is “you.” If I put this ping pong ball in my mouth... is it me? No... it’s just a ping pong ball in my mouth. But what if I swallow it, which I will not, by the way, be doing... would it be part of me then? I think we’d get various answers here but I’m going to venture to say that, no, it’s not part of me.

Likewise, if you weigh 99 pounds and eat a one lump of copper, I’d tend to say that you are not 1% copper. In fact, your digestive system is just, sorta, not... inside you. It’s a long, complicated, tightly controlled tube of outside that runs through your inside.

When embryos first develop, one of the first things they do is pinch a hole down the middle of the spherical cluster of cells, making the cluser a donut shape. That middle hole is, of course, outside the embryo... and it becomes the digestive system. But if you take out all of the stuff in your digestive system, there’s quite a lot of you that isn’t you, including the several kilograms of bacteria that live inside your gut.

By mass, the average human is about 3% gut bacteria. In the end, after digestion, about a third of your nachos will be in your feces within a day or two, and the rest of the atoms and molecules will be breaking down more slowly to feed you, some of them expelled in urine and feces, others expelled in the CO2 that you exhale. But the amazing thing is, by simple virtue of the fact that there are so MANY atoms and molecules in a pound of nachos... SOME of those atoms will remain with you forever, ensuring that you are at least some percent nacho for the rest of your life.

On to the side dishes! And great conversation starters as well--does asparagus make your pee smell? Are you sure?? Here’s The Truth about Asparagus and Your Pee.

You’ve probably never smelled a skunk and wished that you could reproduce that lovely odor with your own bodily fluids. But if you’re among a certain forty percent of us, you actually kind of can. All you’ve gotta do is eat some asparagus, wait a little bit, and then pee on your enemies.

Asparagus is a weird little vegetable. It’s a monocot, meaning that its seeds start out with only one leaf, and it’s related to onions and garlic. Its flowers are actually poisonous to humans, but the stems have plenty of nutrients in them, like vitamin K and vitamin C. And since as far back as the 1890s, scientists have been trying to figure out why it makes some people’s pee smell like... you know, just different. Maybe a little worse than usual.

Of course, all sorts of foods, like coffee and garlic, can change the chemistry of your urine. Which makes sense; some foods contain strong-smelling compounds that your body can’t digest completely, and they’ve gotta go somewhere. Sometimes, that somewhere is your urine.

And asparagus has its own stinky chemical that doesn’t seem to be found anywhere else. It’s called asparagusic acid, and when it’s digested, it produces sulfur compounds like methanethiol and dimethyl sulfide. Sulfur, of course, is famous for giving things like skunk spray and rotten eggs their unique aromas.

But the weird thing is that, for a lot of people, eating even a lot of asparagus doesn’t seem to have an affect on their excreta. It smells totally normal. So when scientists started trying to figure out why that is, they figured that there were two possible explanations.

One was that people who claimed to not have smelly pee were non-excretors - that is, they just didn’t produce urine with the compounds that smell. But the other possibility was that they were producing those compounds, but they just couldn’t smell them. To get to the bottom of the issue, in 1956, two biologists at Oxford conducted a study in which 115 people were fed asparagus, and then had their urine analyzed.

The results showed that some samples did appear to smell, while others didn’t. And the urine specimens that had the unique scent all contained the same compound: methanethiol. Since then, other studies have found even more byproducts of asparagusic acid in excretors’ urine, like dimethyl sulfide and dimethyl sulfone. So it seemed pretty clear that people were either excretors or non-excretors.

But then... we kept studying it. In 1980, another researcher, this time in Israel, began to wonder if everyone’s urine did smell after eating asparagus, but some people just couldn’t smell it. (A really astonishing amount of research has gone into this question.) Anyway, he managed to get 307 people to agree to sniff different concentrations of one person’s smelly asparagus pee. And it turned out that he was sort of right - some people could smell the scent, but others who were given the same specimen couldn’t smell a thing. So... everyone was pretty confused for a while.

Then, in 2011, the Monell Chemical Senses Center in Philadelphia tested both possibilities at the same time. Their subjects gave urine samples before and after eating asparagus, and also had to smell different samples from other people who had eaten asparagus.

The researchers concluded that both hypotheses were essentially correct. It turned out that people were either excretors or non-excretors - AND they could either be smellers or non-smellers. So, some people produce smelly urine but can’t smell it, and some people produce the smelly urine and can smell it (I’m one of those), and some people don’t produce it, but they can detect traces of asparagus in other people’s pee.

Scientists have gone on to determine that all of these differences are genetic. Non-smellers have something called a specific anosmia, where one of their four hundred or so smell-receptor genes is mutated, and effectively turned off. This same kind of mutation is what makes some people unable to smell things like vanilla or mint.

If you’re going to have one of those turned off I’d prefer the asparagus pee one than the vanilla one; that’s just a bummer. The non-excretors, meanwhile, have a different genetic mutation that keeps them from excreting the compounds in the first place. But no one’s really sure where the compounds go if they aren’t excreted.

So it seems that there’s even more work to be done into the science of asparagus pee. Maybe you’ll be the one to do it! French author Marcel Proust once said that asparagus, “as in a Shakespeare fairy-story, transforms my chamber-pot into a flask of perfume.” And we’re all about curiosity here at SciShow, so: does asparagus turn your chamber pot into perfume? Let us know in the comments below.

Now, while the main course is cooking, a lot of people enjoy some physical activity outside. And what better way to celebrate the season than by scientifically smashing its most famous gourd? Here’s an old video on The Physics of Punkin Chunkin.

Are you like me? Do like watching fruit explode? And do you also enjoy physics? If so, there’s probably only one place in the world where these two great tastes come together: Punkin Chunkin! America’s annual contest to see whose home-made machine can hurl a pumpkin the farthest.

Every November, thousands of amateur engineers -- equal parts Leonardo Da Vinci and, I dunno, Gallagher -- converge on a farm in Delaware to put their contraptions to the test. They compete in divisions based on what kind of machine they’ve built, and whoever’s device hurls their squash the farthest wins. The only rule? No explosives.

I know, I’m disappointed too. But don’t underestimate the power of physics -- when its awesome powers are unleashed, it can make even a seemingly simple machine cause a lot of squash damage, and look good doing it. Most machines at Punkin Chunkin rely on the release of stored energy to get the pumpkin airborne. This energy comes from some external force that’s exerted on a part of the machine, where it’s pent up as potential energy.

For example, there are air cannons, which use the potential energy of gas that’s been pressurized. And there are what the contest calls “centrifugal machines” where a motor spins a pumpkin around on a tether -- even though centrifugal force is NOT A THING! But from where I stand, the most delightful punkin-chunkin physics are found in the class of machines that you probably know as catapults.

Yes, they’re not just for castle-smashing anymore! Or, apulting cats [meow]. There are actually many different kinds of catapults -- including mangonels, trebuchets, and torsion machines -- each using a slightly different sort of force to create potential energy. When you think of a catapult, what you probably picture is a mangonel. Its main feature is just a long beam with a bucket on one end and the other fastened to an axle.

The bucket-end of the beam is usually attached by a sling to the machine’s main frame; then it’s pulled down to apply the force of tension. This tension is what stores up the machine’s potential energy until it’s released. For Punkin’ Chunkers, catapults can use springs, cords, rubber, or dead weights to apply tension.

But they’ve added an extra touch of genius to the traditional mangonel -- instead of a just a bucket on the end, it has a sling. What’s the genius in that? It gives the machine a second fulcrum, or pivot point. The first fulcrum is where the beam meets the axle -- when that’s released, it throws up the sling, which amplifies the power of the first fulcrum.

This is all very similar to how trebuchets work, but they’re a little more sophisticated -- like, as catapults go. Rather than holding all of the potential energy at the tip of the beam where the pumpkin is, trebuchets rely on a counterweight on the other end. Again, the beam is pulled down to set the device, but instead of tension being the force at work, here it’s simply gravity. The heavier the counterweight is and the higher you set it, the more force you get out of it when it falls. As a result, trebuchets are much more efficient -- they tend to shoot a lot farther and are more accurate, giving you more chunk for your buck.

Finally, you have what hard-core chunkers call torsion machines. Most of these kinds of catapults are hybrids of the first two -- they have a sling at the pumpkin end like a trebuchet, but instead of gravity, they use the force of tension like a mangonel. Unlike with a mangonel, though, tension is applied to this machine by twisting a length of rope around the bottom of the beam. It’s the same idea as twisting the rubber band on the propeller of a toy airplane -- the twisting, or torsion, is just tension applied in a rotating direction, instead of straight up and down.

You know what happens from here.

Okay, we’ve covered the big questions about the really important stuff: nachos and pee and pumpkin-flinging. But probably the most popular question being asked today is “white or dark meat?” So I have another question for you: why do birds have white and dark meat? What is it about their bodies? And... do humans have white and dark meat as well?

Micheal: If you've ever been in charge of serving turkey at Thanksgiving, you may know that birds have both white and dark meat. That’s because they have two main types of muscle fibers: slow twitch and fast twitch. As you might expect, slow twitch muscle fibers contract more slowly. They’re built for endurance activities, like long-distance flying, and they’re full of the protein myoglobin, which is a special kind of nutritious, salty slime your muscles have to carry oxygen around.

The cells of slow-twitch muscle fibers are packed with mitochondria, the little organelles that act like power plants. The mitochondria take in the oxygen from the myoglobin and metabolize it into a really important molecule called Adenosine Triphosphate, or ATP.

ATP is the fuel, it’s the molecule that stores the energy we use to do just about everything. This process of turning oxygen into ATP is very effective; and as long as there’s oxygen available, the mitochondria can keep it up all day. But it isn’t fast.

So birds also have fast twitch muscles, which are at rest most of the time, but can burst into action when needed. Fast twitch muscles don’t contain myoglobin; in fact, they don’t even contain mitochondria. They couldn’t use oxygen even if it was available. Instead, those muscle fibers produce glycogen, which is a type of sugar that can be burned in an emergency.

Both myoglobin and mitochondria are pigmented: myoglobin is red, and mitochondria are brown. So slow twitch muscles look darker, redder, and browner. Glycogen, on the other hand, is pretty much colorless. So nearly-flightless birds that only ever fly short distances, like turkeys and chickens, have light-colored, fast twitch muscles in their breasts and wings. But long distance fliers like ducks or geese pretty much only have slow twitch muscles. That’s why their meat is dark and gamey.

Now, I know what you’re thinking: do I have light and dark meat? The answer is no... sort of. But also kind of yes... sort of. Most animals have muscles that are either dark or white. In fish, for example, you'll usually find white meat because they float, they don’t really need to use a lot of muscle power just to get around.

But some fish like tuna and salmon have darker meat, because they’re constantly swimming through a current. Mammals however, are different; instead of having some muscles that are slow twitch and some that are fast twitch, almost all of our muscles contain both kinds of fibers. The concentrations of each kind can vary depending on where the muscle is and what it does. The muscles in your eyes, for example, have more fast-twitch fibers, whereas ones in your back that maintain your posture have more slow-twitch.

But the combination of the two fibers throughout your body is why, instead of having brownish muscles and white muscles, we’re nice pink all the way through. That's what we call red meat.

I could really go for a steak right now.

Hank: None of these videos have been particularly good for dinner conversation, I apologize for that. But now that we’re done with the main course, I've decided to keep the trend alive. So: why do we burp and fart?

It’s not exactly dinner time conversation, but it's probably not dinner time unless you watch SciShow while you're eating dinner, in which case, I applaud you. But let’s get it out there: we all burp, we all fart, sometimes we do these things loudly, and often times they smell. But why DO we do these things, and why do some foods and drink exacerbate the problem?

Formally referred to as belching and flatulence, these two often-lampooned bodily functions actually share a common cause: swallowed air. Every time you swallow, you take in some air. But you can swallow even more air than normal when you eat or drink too quickly, chew gum, smoke, drink carbonated beverages, or even wear loose-fitting dentures.

The majority of swallowed air comes back up in the form of burps. As the air builds up in the upper portion of the stomach, it causes stretching that eventually triggers the lower esophageal sphincter to relax. The result is escaping air up the esophagus to the mouth, where the sound and smell of that burp often depends on much Coke you just chugged. Babies in particular are big burpers, especially for their small size, mostly because they tend to gulp in too much air while nursing. And because young digestive systems haven’t developed to the point where babies can easily burp on their own, it’s up to mom and dad to pat their backs and help get those gas bubbles out.

But for babies AND adults, what about the air that isn’t burped out? It passes from the stomach to the small intestine, and later, the large intestine. Along the way as it passes through the gastrointestinal tract, that air mixes with the products of some good old bacteria fermentation that’s going on in your guts. That fermentation is the result of carbohydrates like sugars and starches that can’t be digested by enzymes in the small intestine. Some foods that include these hard-to-digest starches include cabbage, cauliflower, beans and bran, which explains why you may feel excessively gassy after that big bowl of chili.

When those undigested carbs reach the colon in the lower intestines, bacteria take over. The byproduct is a combination of gases that include carbon dioxide, hydrogen methane, and even hydrogen sulfide - a stinky one. That mix of gases -- plus the air that you’ve swallowed -- is what makes its way through the large intestine to the anus, where it is expelled.

Of course, everyone has different amounts of various bacteria and yeasts working away in their guts, but I’m sure you’re DYING TO KNOW, so I’ll just tell you: the average human toot contains roughly 59 percent nitrogen, 21 percent hydrogen, 9 percent CO2, 7 percent methane and 4 percent oxygen. It’s the last one percent -- in the form of hydrogen sulfide and other sulfur compounds -- that’s the most potent in terms of producing those notorious, unpleasant rotten egg smells. While some of these chemicals are produced by bacteria in your gut, eating sulfur-rich foods like eggs, onions and beans doesn’t help on the smelly front.

Finally, it’s time for dessert. And for that, we turn to a video we made on another holiday, Pi Day! But this video isn’t about the number, it’s about the science of that delicious dessert, Pie! With an E!

In honor of this Pi Day, we thought we’d talk about pie. No, not pi, pie! Tender crust with fruit inside. The filled dessert that is superior to all the other desserts. That pie! From the crust, to the filling, to the presentation, there’s a science to making the perfect pie.

Let’s start with the crust. The best pie crusts tend to be light, crisp, and above all else: flaky. And the key to making a light, crispy, flaky pie crust? Vodka. Normally, two of the main ingredients in pie crust are wheat flour and water. But wheat flour contains two proteins -- gliadin and glutenin -- that can combine to form a delicate pie crust’s worst enemy: gluten. The proteins combine when they’re exposed to water, and the more you work the dough, the more gluten will develop.

Generally, more gluten is a good thing -- if you’re baking bread, that is. Gluten is sticky and elastic, and it makes bread dough chewy. But gluten also makes pie crust tough. So, what you want is a pie crust dough that sticks together enough to roll out properly, but stays flaky. And replacing about half of the water with vodka turns out to be a great way to do that.

Vodka is 40% ethanol, and those proteins that make up gluten won’t dissolve in ethanol the way they do in water. But, since the ethanol still adds moisture, you can still work with the dough. Most -- though not all -- of the alcohol will cook off in the oven, so you won’t end up with a particularly boozy pie crust... just a delicious one. Then there’s the filling, which you probably don’t want getting all runny and soggy.

Problem is, fruit has a lot of water in it, and plant cells tend to break down when you cook them, and that lets that water loose inside your pie. One ingredient that can help keep the water from escaping is something that occurs naturally in the fruit itself: a carbohydrate called pectin, which normally helps hold cell walls together.

In pie filling, pectin will form a jelly-like mesh that holds water in place. So, more pectin means a thicker pie filling, which in turns means a pie bottom that is less soggy. For thicker pie filling, you can choose fruits with more pectin, like apples. You can even add an apple to a pie made with low-pectin fruits, like strawberries, for more thickening power. And, a little acid will make pectin stronger and more stable, so some bakers add lemon juice to pie filling.

However, heat destroys pectin, so even a pie with lots of pectin can turn to mush if you overcook it. Finally, you can improve both the look and flavor of your finished pie with what’s known as the Maillard reaction -- aka the most delicious chemical reaction of all time.

It was named after an early twentieth century French chemist who was trying to figure out how to make proteins. And it’s actually more than just one chemical reaction -- it’s a whole category of them. The Maillard reaction is what happens when proteins react with certain sugars at around 140 degrees Celsius. The reaction can form all kinds of different products, depending on the proteins and sugars involved, but generally, it’s what makes bread develop a crust, and cooked food turn brown. It’s also usually considered something that makes food extra-delicious, and there’s an easy way to help the reaction along in your pie -- or pretty much any pastry, for that matter.

All you have to do is brush some egg white over the crust while it bakes. The protein in the egg white will react with sugars in the crust, setting up a Maillard reaction and giving your pie that deep golden brown color. So, happy pi day everybody, and we hope these tips help you impress your friends with a Pi pie.

Thanks for watching this special SciShow Thanksgiving compilation! We’re especially thankful, of course, for our Patreon Patrons who make all our videos possible. Thank you! Now, go get yourself a slice of pie, or some nachos, or whatever you like! And don’t forget to go to YouTube.com/SciShow and subscribe.