why does still water freeze below at the freezing point?

Moving water does freeze. Sort of. Frozen water is crystalline H2O. It forms when the water reaches the right temperature. (which actually varies, but is about 0 degrees Celsius) So moving water (like in a river) freezes into very tiny separate crystals, rather than one large crystal (like in a bucket). If the water gets cold enough, the crystals get so "sticky" that they bond to each other anyway, and flow as large chunks, or impede the flow entirely.

Freezing does convey the water molecules close jointly. yet because of the shape of water molecule, a void is created in the course of the formation of the ice crystal with air trapped interior. hence its density decreases and volume seems to have more desirable. an similar reason can clarify cases like why ice floats on water(with the aid of low density) or why a lota's structure( seem for the percentin google) ameliorations at the same time as that's fillled with water and put in a freezer.

The chemical process taking place is a binding of water molecules together. When water freezes the molecules next to each other attach, it takes a little while, you wouldn't want to stand out in the cold and watch water freeze.

In order for two molecules to cling to each other they have to be still enough to allow the process of binding to take place. If the water is moving, the molecules are all bouncing and jumbling around so quickly that they can't attach. Thus you can have water right on the brink and just below freezing that is not actually frozen.