We’ve all been taught that water freezes at 32F (0C) but in actuality water can remain liquid below this temperature, under special conditions.  Imagine you’re sitting on your couch after a long day, you’re tired and you’re starting to feel a bit hungry, but the fridge is so far away.  Eventually you get hungry enough to get up, go to the fridge, and satisfy your hunger.  Water can relate.  When the temperature of the water drops below 32F it would prefer to be a solid, but it takes energy to change from a liquid to a solid.  As the temperature gets lower the water gets “hungrier”, it wants to be a solid even more, eventually it wants to be a solid enough to overcome the energy barrier, the “walk to the fridge”.

When water wants to become a solid there are two ways it can go.  It can either grow on a surface, like condensation on a cool drink or around a dust particle like rain, or, if it has enough energy, it can grow little spheres of solid within the liquid, without the help of a surface.  The amount of energy required for this phase transformation is directly related to the amount of surface created.  When the transformation is happening on, say the inside surface of a water bottle, the liquid only has to support the area of a dome, the bottle takes care of the rest. When there is no bottle to work with, or the liquid is far from the surface of the bottle, it has to have enough energy to support the surface area of a whole sphere.

If you’re trying to replicate the video above it’s crucial to have very clean water.  This means there are no little particles that the water can use to lower the amount of energy required to freeze, it has to save up enough to grow the spheres without any help.  The water also needs to be cooled slowly and handled gently because any significant energy changes, thermal or kinetic, can give the water enough energy to start freezing.  This is why hitting the bottle will start the reaction.  When the bottle is hit, it finally gets the “oomph” it needs to get to the fridge (freeze).

These phenomena (heterogeneous and homogenous nucleation) are also responsible for the famous Mentos and Coke experiment, why bubbles in carbonated drinks seem to come from specific points in the glass, and how engineers make aircraft aluminum strong enough to keep planes in the sky.