Why does ice float on water ?

"Ice is less dense than water because of its intermolecular forces."

Water contains hydrogen bonds (a type of intermolecular force of attraction) between the H (hydrogen) of one atom and the O (oxygen) of another atom.  As the water gets colder and the kinetic energy of the molecules decreases, the hydrogen bonds keep the water molecules apart, forming hexagonal structures with water molecules at each vertex.  In between the water molecules is nothing.  In liquid water, the molecules of water can be much closer together; the hydrogen bonds are more flexible.  Therefore, the solid ice, with its molecules kept at a fairly fixed distance and the crystals holding lots of "nothing" among the water molecules, is less dense than the liquid water.