​​Why sometimes Water freezes instantly with just a shake?

 ​​Why sometimes Water freezes instantly with just a shake?

The basic idea is this: when water freezes it forms ice, which is a nice regular crystalline structure. However, ice-crystals need a nucleation site, which is a point where the crystal can start to form, before they can actually start to form. In normal situations, water usually has some impurities which can serve as such nucleation sites, around which the crystal starts to grow and ice starts to form.

However, if you use very pure water, there are no such "natural" nucleation points and so there is a chance that the water molecules want to form ice, but can't quite get around to it. As a result, the liquids are trapped in a "metastable" state well below their freezing point, but such a state has a precarious stability that can easily be disturbed. Shaking the bottle is one way to disturb this stability, as it gives a couple of the water molecules the chance to align in just to right way to start the crystallisation process, and once it's done, it is energetically favourable for the system to form ice, so all the other water molecules hop on as well. As a result, you would usually see the crystal "growing" in one direction until all the water becomes ice.