You know this is true, that ice is lighter than water, because you can see ice cubes floating in a glass of water. The reason it expands is that the molecules of water re-arrange at the time of freezing to form the crystal structure of ice. This expansion takes place with tremendous force, as anyone knows who has left a full container of water with a tight lid in the freezer. The force is enough to burst the strongest water pipes if the water in them freezes, which is why people in cold climates sometimes leave a little water trickling through the pipes on freezing nights. It is not possible to make a usable pipe strong enough to withstand this force.
Freezing water can burst a cast iron pipe over a foot thick.
Water expands when it freezes because the molecules of water are actually closer together in the liquid state than they are in ice. Water molecules are polar, which means that they have positive and negative ends like little magnets. When water freezes, the molecules are held together in a rigid crystal pattern called a lattice , lined up with opposite charges next to each other. When the ice melts, the water molecules can tumble over each other and briefly get a little closer than they are in the solid crystal. This expansion is a very important phenomenon in the natural world.