Water molecules move about freely when in the liquid state. There is little space between the molecules as they zigzag about the place taking up the shape of the container.

When cooled, the water molecules have no energy with which to move about and are attracted close to each other. The molecules have little charges on each end which now take effect and attract each other into fixed, rigid positions. This forms an unusual configuration with a great deal of space in between the groups of water molecules. This causes ice to expand as more room is required which forces the walls of the container outwards.

The spaces that form in amongst the water molecules make ice less dense than water. As a result ice floats on the surface of water.