Try telling someone who has just fallen on a patch of ice that ice is not slippery, and they'll think you're crazy. But, in fact, ice itself isn't slippery because it is a solid. One quality of solids is that when two solids are together, there is friction between them and that will keep them from slipping. So how can your shoes slip on ice? The answer lies in two peculiar properties of ice. The first is that as water freezes, its molecules move farther apart. The molecules of most substances move closer together as they freeze, making them shrink at lower temperatures. But water molecules move farther apart at temperatures below 39 degrees Fahrenheit, making water expand as it freezes. That is why frozen water pipes burst, and a tray of ice cubes will freeze over its top if you fill it too full. The second peculiar property of ice is directly linked to its first peculiarity. When subjected to pressure, ice melts. Remember that the molecules in ice are farther apart than the molecules in water; therefore ice molecules are vulnerable to pressure which pushes them closer together, causing the ice to change into water. So when you step on a patch of ice, you exert pressure on the ice, which causes its molecules to move closer together. That makes them revert to their more dense state, which is water. If you slip on a patch of ice, then, you in fact are slipping on a thin layer of water that the pressure from your weight has created. And, unlike solid ice, water, as a liquid, is quite slippery.