When I was growing up in the 1930’s, all cows were inspected for disease and if they were healthy, farmers just milked them, poured the milk into glass bottles, put caps on them and delivered the bottles to door-steps every morning. I very clearly remember when I was sent out to bring in our bottles of milk in the winter, I often found that the milk had frozen and had sprouted an amazing column of frozen cream on top [no homogenizing those days].
It was only much later that I finally began to really appreciate what a unique property it was that water expands when it freezes and I got that nice jolt of pleasure when I understood why. Imagine some of the implication if water didn't expand and float! Rivers and lakes in colder regions would freeze from the bottom up and and all the marine life where that happened would be frozen solid.
Molecules are always on the move and, at higher temperatures, they move faster and tend to mostly bounce off one another without sticking. However, as the temperature goes down, they slow down, stick to each other more and the water gets closer packed and denser so it shrinks. That happens to all materials but something pretty unusual happens to cold water at 4 degrees C [39oF]. At that temperature the water molecules are moving so slowly they start sticking together and connecting to form an open hexagonal pattern that spreads them apart making the water expand. The video representation is supposed to represent water cooling until it forms the open hexagons.
In 3D the pattern is more like a honeycomb and you can easily see why it would float. Warm up the ice, molecules move faster, the weak attractions between them break and the whole structure collapses to liquid water again.
If you have read this far and are interested in things like this, maybe you should have been a scientist – nature can be endlessly fascinating especially at the molecular level. Rie