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Hank: Hi, I'm Hank Green and this is SciShow. This is our 790th video! That's a lot of videos, so many that very few people have seen them all, in fact, I don't think I've seen them all. And we get a lot of requests for videos topics that we've already made. Now a lot of you have been here from the beginning and you are usually the first to point out where people can find the topics they're interested in and thank you for that.

But we wanted to try something new as well. While some of you have been requesting videos we've already made, others have asked to watch playlists without credits and intros and pre-rolls interrupting you every four minutes. So we are combining those two needs into this, an extra video about once a month that's a compilation of older videos we think you will enjoy. They might be recent requests or a group of videos answering similar questions and topics. We might even add some commentary in between videos, let us know what you think we should do.

In the meantime, we've been getting a lot of requests for winter related topics, so here are some of our favorite videos we've made about how fake snow is made, why we can see our breath in the winter, and killer ice cubes of death, just to name a few. Most of them are hosted by me, but there's a pretty cool experiment in here as well, hosted by Michael Aranda. Enjoy.

If you enjoy winter, then you probably like all the stuff that winter makes possible, like skiing, sledding, making snowmen and building snow forts. But to do any of these things, you need snow! And winter does not always equal snow. What if there's not as much snow at your local ski hill as there usually is? Or what if you're hosting the Winter Olympics at a resort on the Black Sea? And what if you also don't have a sister who was born with the freakish magical power to freeze anything she wants? Then you have to make the snow, the non-magical way, the scientific way. And the science of snow-making is more complex than you might think.

Snow is precipitation composed of ice crystals. Unlike sleet or ice pellets, which is liquid rain that freezes on it's way down to the ground, snow forms when water vapour skips the liquid stage and condenses directly into ice, in clouds where temperatures are below zero degrees.

The ice crystal then grows into a snow crystal or snow flake as it absorbs and freezes more water vapor. So making snow is basically all about creating a heat exchange because heat has to be removed from water in order to transform it into ice crystals. For this to happen the ambient air temperature, the snow-makers call this the Dry Bulb Temperature, has to be really cold - even at zero degrees any manufactured snow is likely to be low-quality slush.

But equally important is the humidity, the amount of water vapor in the air. When the ambient temperature is adjusted to account for humidity it's known as the Wet Bulb Temperature. This is important because water cools by evaporating into water vapor. Your body uses the same process to cool itself, we call it sweating. 

So if it's too humid and the air is too saturated the water can't evaporate as well, which makes it harder to cool down. So snow makers hate high humidity almost as much as they hate low temperatures. They need both low air temperature and low humidity. When these conditions are at their sweet spot most snow machines, or snow guns, at your favorite ski area can combine cooled water and compressed air to create tiny water droplets and the smaller those droplets are the better because lots of little drops expose more of the water to the cold than few big ones. So snow guns use special nozzles to spray a fine mist that, snow gods willing, freezes almost immediately. To boost their chances most snow machines also shoot that mist high up in to the air or even use elevated snow lances or snow gun towers. These allow the super chilled water droplets to stay in the air longer so they have amble time to freeze and become that wonderful white stuff we need for skiing.

So it's not done the same way nature does it and it's not as easy as pointing finger at whatever you want to freeze. But the science is almost as fun, and frankly I don't care where it comes from or how it's made. I just want my snow because I want my Olaf!

You may have noticed that when it’s really cold out, your skin seems to get sticky. Touch your finger to a piece of ice, and suddenly it’s like it’s glued to you. Touch your tongue to a cold flagpole outside of your school, and you’ll become a legend. But not in a good way. They’ll probably dedicate a whole page of your yearbook, just full of embarrassing pictures from that fateful day. 
  
But why? Why did your tongue stick to the flagpole? The reason is actually really simple, even if the effects aren’t pleasant: There’s saliva between your tongue and that flagpole, and it freezes on contact. The newly-formed ice acts as a bond between tongue and flagpole, almost like superglue. And unless you figure out a way to melt the ice -- like by pouring hot water on it -- the only way to separate that tongue and that flag pole is through the path of least resistance. And your tongue is gonna put up less resistance than the flag pole. 
  
The brute force method can get... messy. And painful. But here’s the thing: Why is the flagpole on your 'Do Not Lick' list for winter, but not, like, wooden telephone poles? Well, you only freeze to certain types of materials -- specifically, the ones that are best at conducting heat. Let’s start with the fact that the inside of your body, even in winter, is always nice and warm. That’s because your body is doing its best to keep your organs and fluids at around 37 degrees Celsius, even when parts of you are exposed directly to frigid air. 
  
This is, for example, why your eyes don’t immediately turn into eyeball-snow cones in cold weather. The warmth of your tissues and the blood flowing through the vessels in your head keep them and other parts of your face from getting below freezing. And most of the time, this same heat-generation is happening on the surface of your skin, including your tongue.
  
As long as your body can heat up your skin as fast as that heat is transferred away to something colder, it’ll stay above freezing, but things like metal and ice are good conductors of heat, and after a short time, your body can’t keep up. In the case of the flagpole, the metal is able to siphon the heat out of your tongue faster than your body can replenish it.  So your skin gets below freezing pretty quickly, and the saliva turns into ice. 
  
But since other materials, like wood, aren’t good conductors, your body can replace the heat fast enough to keep your tissues warm. So, if you want to lick a cold, wooden telephone pole, you’ll probably just end up with splinters on your tongue. But if there were a doctor or a lawyer in the room, they’d probably tell you just to not go around licking stuff... summer or winter. It’s gross and dangerous, and the whole reason I’m telling you this is so you don’t end up being labeled 'Most Likely to Lose the Tip of their Tongue' in your school yearbook.

Michael Aranda: At standard pressure, water freezes at zero degrees Celsius and boils at 100 degrees. Standard science, right? I mean, the freezing and boiling points of water are how we define the entire Celsius temperature scale. Except that water doesn’t always freeze when it’s supposed to. 

When the conditions are just right, you can put a bottle of water in the freezer for hours, and it won’t turn to ice... until you make it. That’s because liquids are subject to a special, weird state known as supercooling, when the liquid is below its freezing point, but doesn’t solidify.  Liquids don't just suddenly become solid at a certain temperature. There's a process involved, and it has to do with how molecules line up. Whether something is a solid, liquid, or gas at constant pressure mostly depends on its energy, which comes from heat.  
  
In a gas, molecules are warmer, meaning they’re higher in energy. They bounce around a lot, colliding with each other and flying every which way. But in a liquid, molecules don’t have as much energy. They’re still moving around, but they're slowed down enough that bonds form between them, keeping them close together, and loosely ordered. Now, at even lower temperatures, where molecules have the least amount of energy, they form a solid. They’re still vibrating a bit, but at this point the bonds between them are so strong that they keep the molecules rigidly in place. 
  
Now, let’s talk about the water in the bottle. To freeze, the water needs two things: It needs to have low energy - that is, it needs to be cold - and it needs to form the intermolecular bonds that are going to hold it together as a solid. For this second part to happen, the water has to first form a nucleus, or central point, of what will eventually be solid ice. This is called a substance’s nucleation site, it's where the water molecules first slot together in exactly the right way to form the structure of the ice crystal. This then allows other water molecules around it to form bonds, spreading out until they all make one solid piece of ice. 
  
However, nucleation sites don't usually happen by themselves; something needs to cause them.  And, often, impurities in water act as nucleation sites. A tiny deposit of some mineral, for example, could be enough to nudge the molecules around it so they start to arrange themselves into a solid. Or sometimes, a nucleus can form if the water is physically disturbed somehow -- say, shaken up a little. As the very cold water sloshes around, some of the molecules will slot in next to each other in just the right way, so the water is able to start the process of turning into ice. 
  
So if you want to supercool water yourself, you have to keep both of those things -- the impurities and the movement -- from happening.  All you need is a freezer, a watch, and a bottle of standard drugstore-brand purified water. Don’t use mineral water, because that stuff is full of things that will act as nucleation sites. 
  
First, just stick your bottle of water in the freezer. We do want it to be below freezing, but the longer we keep it in there, the more likely the molecules will find a nucleation site. So around two and a half hours should do the trick.  During that time, don’t open the freezer! The cooling needs to be gradual, and the change in temperature when you open the door disturbs the water enough that it might find a nucleation site. So just leave it alone. If all goes well, when you take the out bottle out, you’ll have yourself some supercooled, below-freezing liquid water. But, if you shake it up... 
  
Supercooled liquids are highly unstable. When they’re below their freezing point, their molecules REALLY WANT to form a solid. Like, REALLY BAD. The laws of physics are commanding them, but they just can’t comply. And that is why, with just a slight provocation, those supercooled molecules will snap into place. By slamming a bottle of supercooled water against the table, you create a nucleation site, and you can actually see the ice crystal form almost instantly. 
  
Which is SUPER COOL! Get it? Supercool?

Hank Green: Absolute zero is the Holy Grail of temperatures, but even though we know exactly how cold that is (-273.13°C (-459.670°F)), reaching it has eluded us for centuries and will continue to elude us, probably forever. 

For this frustration, we have to thank Sir William Thomson, 1st Baron Kelvin, who, in the mid 1800s, tested a new theory that heat is just molecules moving around in a substance. So, he wanted to get stuff as cold as he could get it, so he conducted experiments that drew heat from a warm substance toward a cooler one, and found that at some point, all the kinetic energy could be drained from the warm substance -- it could no longer be cooled any further. 

This temperature wasn't like melting points or boiling points, which change for every substance. It was the same for everything. So Kelvin created a thermodynamic temperature scale that measured the amount of kinetic energy within any given material, and we still use his Kelvin scale today. 

But ever since Kelvin's day, scientists have been trying to chill stuff to absolute zero, and no one has succeeded -- all a bunch of failures, because it turns out quantum mechanics is involved, which means it's really complicated. 

Physicists know that absolute zero does not mean a complete absence of motion in a substance. Instead, zero degrees Kelvin marks the state of minimal motion of a substance's particles. That's because of Heisenberg's uncertainty principle, which says that for any particle in the universe, it's impossible to know both its momentum and its exact position at the same time. 

So, suppose you chill a lump of lead down to the point where there is no motion going on within it, even at a subatomic level. If you could do that, you'd know both the particles' positions and their momentum, which would be zero, but measuring this is impossible -- it's forbidden by the uncertainty principle, so it cannot be done. So you can't reach true 0°K but you can get pretty darn close, like a billionth of a degree away. When you get that cold, some pretty weird stuff starts happening. 

Below about 30°K, some substances can become superconductive, meaning that they carry an electrical current with no resistance, which is super useful when you're making particle accelerators or really powerful electromagnets to put in your MRI machines, and those superconductors have been discovered that operate at much warmer temperatures. The development of the field is thanks to work at very, very cold temperatures measured on the Kelvin scale. 

And you might be wondering, because I was, how cold is the coldest place in the universe? You'd think, like, deep space, right? Well, yeah, space is cold, but it's pretty uniformly filled with microwave radiation left over from the big bang. This actually heats up space to a balmy 2.73°K. The coldest natural place in the known universe is the boomerang nebula, which has been spitting out gas for so long that it's cooled down to only about one degree Kelvin, and all of this means that, in fact, the coldest place in the known universe is in laboratories right here on planet Earth. Pretty cool. 

Hank: You've been doing it since you were a little kid. On a cold winter day, you take a deep breath, let it go, and watch big foggy billows come out of your mouth and float away. It's like you're making clouds! But why doing it happen? Why can you see your breath when it's cold out?

Well, it's because the moisture in your breath is experiencing a rapid change in dew point. Dew point is the temperature at which water vapour starts to condense out of the air and forms a liquid just like the dew on your front lawn. When air's warm, like in the summer or inside your body, the dew points are high because heat makes water molecules move around fast and turn in to gas. So warmer air can hold more water, that's why you get so muggy in the summer. But cold air slows these molecules way down and they begin to condense. As a result, lower temperatures often mean less moisture in the air.

Now our lungs, no matter what time of the year, are kinda like Georgia in July – warm and muggy. When you exhale, the air leaving your body is actually completely saturated with moisture. That means the relative humidity of your breath is 100%. And when the water vapor in this warm, sultry, saturated air strikes the cold air, those water molecules slow down and quickly condense in to liquid. It's the same mechanism that causes fog. Masses of warm, humid air striking cold air causing condensation.

But here's the thing: in order to condense, those molecules need something to stick to. So they actually attach themselves to tiny particles in your breath called 'condensation nuclei', or even 'cloud seeds'. These provide a solid platform for water vapour to condense into a droplet and your cloud seeds are composed of all kinds of things. Soot, dust, cell particles from your lungs, even bits of your proteins and DNA. In fact, scientists are developing a way to analyze our exhaled breath condensate, or EBC as it's called, to look for the presence of air pollutants or signs of respiratory illness. But when you see your breath on a cold day, it's born from the same phenomena that form clouds and fog and dew.

Right about now, below the frozen surface of the Arctic Ocean, a column of salty liquid is flash-freezing the water around it, creating a tube of ice that can grow fast enough to be observed in real time.  As it stretches down, maybe as much as several meters, this fragile frozen dagger will make contact with the seafloor, where it'll create a fast growing web of ice that can trap and freeze any aquatic life that's too slow to get out of its way.  This otherworldly sounding formation has been called an 'icicle of death', but scientists know it as a brinicle, and it is as cool as heck, unless of course, you happen to get caught under one, in which case, it's just very cold.

We've known about brinicles since the 1960s, but it wasn't until a few years ago that we got our first really good look at them, thanks to a BBC documentary that features some of the craziest time-lapse video you will ever see.  They're found in both north and south polar seas, usually as winter approaches and new ice begins to form.  Ocean water doesn't freeze like the water in your freezer or in a freshwater lake, when sea ice freezes on its surface, only the water freezes and the salt and other ions in the water are pushed out, leaving behind brine.

As the water continues to freeze, this salty highly concentrated brine accumulates in tiny channels and fractures within the ice, and when the ice cracks open just a bit, it gives the brine and escape route.  Because of its high salinity, this liquid brine is denser and also colder than the water below it, and since it's so salty, it freezes at a lower temperature than fresh water.  When this super cold, super saline liquid escapes from the sea ice, it begins to sink, and immediately encounters water that's very near its freezing point, but still not as cold as the brine.  And this is where all that amazing flash-freezing starts.

Our briny concoction immediately freezes any seawater it contacts as it sinks, a fragile tube of ice quickly forms around the descending plume and thickens as the brinicle stabilizes.  The process continues even if the formation hits the ocean floor, as the still delicate structure continues to freeze anything it touches.  And that's where things become a little bit dangerous for slow moving critters like sea stars and urchins.  Scientists have reported finding "black pools of death" under active brinicles or in places that used to have them, littered with the skeletons of marine animals that couldn't move fast enough as the ice enveloped them.

The study of brinicles is still in its infancy, there is still much that scientists hope to learn, but let's be patient, arctic water that's negative 2 degrees Celsius, beneath layers of sea ice, it's not exactly the most hospitable research environment.  Still, it might be worth braving those conditions just to see one of them in action, though probably I'm not gonna go check it out.

So this isn't something that e spend a lot of time thinking about but the fact that ice floats on liquid water is really weird and really important; and not just because it lets your ice tea make that nice tinkling sound but because most aquatic organisms depend on it.

If ice didn't float then rivers and lakes would freeze from the bottom up instead of freezing on their surfaces, and all the living things in them would be exposed to cold from both the ice below and the frigid air above. Most living things wouldn't be able to survive a single winter of this.

But fortunately ice does float, so it creates an insulating layer between the cold air and the water allowing organisms to survive and swim ll winter long.

Okay but why does ice do that? 

First we need to know why anything floats or sinks and the answer to that is simply density. If a material is more dense than the fluid that surrounds it it will sink; if it's less dense it will float. So in short the answer is that ice floats because frozen water is less dense than liquid water.

But how is that possible? Isn't it all just water? an aren't slid like definitionally more dense than liquids?

Well not definitionally, usually yes but water molecules are special in the way that they are shaped and how their electrons are distributed around them. So we all know that water is composed of two hydrogen atoms and an oxygen atom but you might not know that oxygen attracts electrons more strongly than hydrogen does. The result is that the electrons in a water molecule hang out closer to the oxygen than the hydrogen's; and this give the O side of the molecule a slightly negative charge while the H₂ side has a slightly positive charge. This makes water a molar molecule and a lot of waters coolest and most useful properties comes from this polarity

So when water is in it's liquid phase it's molecules, like in all liquids, randomly organized but as the water cools the molecules get closer together and at a certain point, starting around 4^oC they get close enough together that the positive and negative areas of those molecules start attracting each other and they snap into a rigid structure. Because water molecules are bent at an angle when they line up like this the structure they form leaves little holes between them which makes the structure less dense.

Those tiny holes are the entire reason solid water floats on top of its liquid form. So remember that next time you skate on a frozen pond or enjoy a nice icy bevvy.

I hope you enjoyed that and now it is time for me and maybe you to go enjoy a nice hot beverage of your choice. Thank you for watching our first SciShow compilation show, let us know what you think and what you might like to see more of in the comments.

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