because of the way it forms a hexagonal crystalline structure. Each water molecule consists of two hydrogen atoms bonded to the bottom of an oxygen atom. When ice forms, the hydrogen atoms of one water molecule form weak hydrogen bonds with the top of the oxygen atoms of two other water molecules. Lining up the water molecules in this pattern takes up more space than having them jumbled randomly together (as is the case in liquid water). And because the same mass of molecules takes up more space when frozen, ice is less dense than liquid water. For this same reason, water below 4 degrees Celsius becomes increasingly less dense as it gets colder. Close to freezing temperatures, the molecules in the liquid water begin to line up into the space-filling hexagonal structure.