Why does ice float?

I've been waiting years for one of my kids to ask me that and really care about the answer. My daughters both inquired at various times but weren't really very interested in the molecular reason that water floats.

Last week my son was concocting pink potions with suds and floating ice in it, and spilling all over the place of course. When he inquired, I could tell he knew he would be interested in the molecular structure of water when it becomes solid. I explained briefly about hydrogen bonding and how the water essentially becomes "fluffy" and lighter than liquid water. He was so interested that I drew the structure for him and showed him where the bonds form between the hydrogen of one molecule and the oxygen of another, forming a triangular structure. He spent a good chunk of the afternoon trying to figure out how to draw the molecules with different types of bonding between them and explaining to me what he was trying to do. I didn't really have a chance to explain how the electrons are shared in order to form these bonds, but I think I may have explained it to him once before so maybe he was basing his ideas of bonding on bits of what he remembered from that discussion.

I've always been a fan of hydrogen bonding in water that causes ice to form on the surface of lakes, rivers and other bodies of water. Although my girls weren't exceptionally interested in the molecular bonding in water, they were able to appreciate that liquid water in the bottom of ponds was a very handy thing to allow fish, frogs, toads, plants, etc to overwinter. Maybe the next time they walked the dog by our neighborhood pond they thought of that as they contemplated the life at the edges.