Okay, Doper physicists, one of you explain how this happens?
I’m thinking something to do with the sudden excitation of water molecules in a extra-cold environment but that’s all I got. And I don’t actually know what that means. 
It’s not that strange.
Many liquids can be supercooled (temperature below their normal freeing point) without freezing for a long time, if there are no “nucleation sites” - bumps or irregularities that facilitate crystal formation. When he hits the bottle, it “disturbs the equilibrium” of the system (basically, I don’t know precisely what is going on), and once the ice starts to form, it provides a surface for new ice to form, and the whole thing freezes rapidly.
I’ve seen bottle soda do the same thing. Leave it in the freezer to long. You take it out and for a moment its not frozen. It turns to a solid as you watch.
You can also get water to do the reverse, i.e. superheating. It’s most easily accomplished in a microwave (note: don’t actually do it, you could get serious burns,)
But if you heat up water (distilled would work best) in a very smooth vessel for a long enough time in the microwave, it might not boil. But then you go to pick it up, and the movement will cause it to basically all boil at once, and essentially explode all over the mug, microwave, and you.
You can also superheat water in a microwave using tapwater and a not-all-that-clean mug. It’s much less likely to happen, but it can.
Weird. I’ve never even heard of the phenomenon with the water bottle. I think I get it because it seems to be the same mechanism that causes a bottle of shaken soda to “explode.”
On the other hand, there have been several occasions where a cup of water heated in the microwave has exploded about as violently the instance I introduced a tea bag. And I was very careful to make the introduction in accord with accepted diplomatic protocol. :eek: 
It’s ice, but it’s not a complete block of ice, more like slush.
That’s because the latent heat of melting of ice is so large (334 kJ/kg). In fact, if you wanted to get a complete solid block of ice, you’d have to supercool the water to -80 °C. If the water was supercooled to “just” 20 °C below freezing, only 25% of the water would freeze.
According to the same physics, it takes as much energy to melt a block of ice as it takes to heat the same amount of water by 80 °C
ETA: If you want to check on the math, you’ll need the heat capacity of water: 4.19 kJ/(°C·kg)
(or 4.19 kJ/(kg·K), since temperature differences are given i K, not °C)
Nucleation sites are involved in both processes, but that’s where the similarity ends. For the shaken soda bottle, you’re dealing with dissolved CO2 coming out solution at the nucleation sites; for superheated/supercooled water, you’re dealing with the water itself undergoing a phase change at the nucleation sites.
If you want to play with supercooled liquids in your own home, you can google “sodium acetate trihydrate” and watch you tube video instructions for making sodium acetate from baking soda and vinegar.
sodium acetate trihydrate is easier to supercool than water.
Adding a Mentos candy to Coke is another harmless experiment too. Do it outside because Coke will spray in huge geysers and you’ll probably get wet. Nucleation makes it work. Google has more info.
Yes. I just wanted to point out it’s not as amazing as it looks on video - a liquid transforming into a solid instantly. It’s more like liquid to Slurpee.
Regular Coke? For some reason, I thought it had to be Diet Coke.
Mythbusters says Coke works too, but Diet Coke is the best. 
I hate when this happens with beer. I’ll put some room temp beer in the freezer and sometimes wait a little too long. It looks o.k. when I take it out but as soon as I open it - beer slurpee.
That makes it look cool when you pour it instead of rapping the bottle.
My elementary school science teacher swore that as a child he did this with an entire pond. He said that he once threw a rock into an unfrozen duck pond in subzero weather and the entire pond froze over, starting at the point where the rock entered the water and radiating outwards.