fact: heat makes stuff expand
fact: cooling makes stuff contract
Why, then, does ice “swell?” 
e.g. Most people know that putting a bottle filled with liquid, into the freezer is a big no-no.
When water forms ice, it goes from a fairly random state (the molecules wiggle a lot) to a far more ordered solid state. It’s not just any solid state, but a crystal: a lattice, or grid, of molecues, all in their special place. The spacing between molecules in this grid structure is larger than between molecules in liquid water, on average. Therefore, water expands slightly when forzen.
Also, this makes ice less dense, so it floats. That’s nice for cool drinks, but it’s also critical for lake life: water freezes from the top down, and not from the bottom up. If ice were more dense, lakes would freeze at the bottom first, and push all the water out as it freezes. It would also freeze more easily, since the floating ice acts as an insulator, protecting the water beneath it. Without that floating ice to insulate them, even deep lakes could freeze solid in the winter. Since ice floats, lakes are protected and don’t freeze solid.
My undergrad years of chemistry are long behind me, but see if this helps:
Water is a polar molecule; i.e., it has a “positive” and “negative” side. This allows weak hydrogen bonds to form between molecules. The normal angle of the two hydrogen atoms in the molecue is 105 degrees, but as water cools it forms into 3-dimensional crystals with hexagonal sides, deforming the angle from 105 degrees, making the crystals take up more room than the liquid, decreasing the density, and resulting in floating ice.
Simple, huh?
Only Ice I (everyday ice, the only kind that forms at ambient temperature and pressures available on Earth) has this characteristic. Ice III, for example, sits at the bottom of the cold, pressurized water like any decent solid ought to.
And only hydrogen monoxide (and hydrogen fluoride) among all compounds seem to have this characteristic.
My first response to the answers above: Thank you.
My second response to the answers above: :eek:
My third response to the answers above: Hi, Opal!