 Okay, pure water. A single water molecule is bonded together covalently, so the hydrogen and the oxygen atoms share the electron covalently. The hydrogen, oxygen, bond, however, is polar. That means the electrons are not equally shared. Since oxygen is much more electronegative than hydrogen, the electron density mostly resides on the oxygen, or you can say that the oxygen pools most of the electron density to itself. This leaves little electron density on the hydrogen. Since oxygen is electron-rich and hydrogens are electron-poor, we call these cars a bond, polar bonds, because there is not an equal sharing of the electron density, which is what a non-polar bond would have. And we indicate the electron-rich region with a delta-minus symbol and the electron-poor region with a delta-plus symbol. The most liquids have a quite simple behavior. When they're cool, they shrink. This is because molecules have much less kinetic energy at lower temperature, so their intermolecular forces become stronger and the molecules struggle to break those forces to move around freely. But when water goes below four degrees Celsius, the water molecules lose most of its motion, so it won't move at all practically, and when things don't like to move, they like to arrange themselves into a crystal structure, forming what we know as a solid. Normally, a solid is the densest form of matter, but there are many exceptions and water is one of them. In the case of water, the water molecules arrange itself into a hexagonal lactase. This lactase, see the spaces here, contains more space than the liquid state. So the freezing point of pure water is zero degrees, but it starts to structure itself into a hexagonal form from four degrees, so when you cool liquid water down from said room temperature, it will shrink, like most liquid would, because molecules move around a lot less until it gets to four degrees, which is where it starts to arrange itself into this hexagonal crystal structure, and then it will expand until every single water molecule falls into the hexagonal lactase, which we know as ice. This is why ice flows on liquid water, because it's less dense. Here is what the lactase looks like in 3D. The fact that ice flows on liquid water has allowed nature to create phenomenal ecosystems around the addicts, where the icebergs would float on top and act as an insulating layer, like a blanket, protecting the liquid water from below, from the cold air on top, and thus prevent water at the bottom from freezing, allowing living organisms to leave below the icebergs. One really spectacular example is Lake Vostok. Feel free to do a bit research on this. Essentially, scientists have found a lake at the southern pole of cold beneath Russia's Vostok station under an ice sheet of 2.2 miles thick, and they are speculating that there are organisms trapped underneath from 15 or 25 million years ago that have been protected by the very thick ice sheet.