Why do ice cubes float?

The fact that ice cubes float on water tells us something important about water. When water freezes and changes into a solid, its molecules arrange themselves so that they occupy more space.

Another way to think about it is that a quart of ice weighs less than a quart of water. The ice is lighter, so it floats. Most liquid substances contract and occupy less space when they change to their solid forms. For example, a piece of solid lead sinks if dropped into a pot of hot melted lead.