Why does an ice cube float on water? I alrdy know that ice is less dense than water.?

particle theory? I thought it was just that when molecules of water slow down, a lattice is formed creating a less dense ice cube than its surrounding water molecules.

Are you wanting to know why, IN GENERAL, less dense objects float in denser fluids? Or why ice in particular is less dense than its liquid water counterpart? In GENERAL, less dense objects float on denser fluids because they are able to displace their own weight worth of the fluid, receive an upward force of buoyancy fully opposing its weight and still have some volume kept above the fluid surface so that it isn't fully immersed. Ice and water in particular are a very unique combination of solid and liquid for the same substance. Most substances contract in density upon freezing. Water ice is UNIQUE, and by contrast freezes into a special crystalline structure that supports voids of empty space in the hexagonal crystals of H2O molecules. Liquid water has the intermolecular bonds mostly broken, so that there are no significant forces preserving the void. There are very few other substances that exhibit this property. Here is a diagram http://img683.imageshack.us/img683/8721/uniqueice.gif

Oxygen bubbles are trapped in the water like little ballons.