Another pop quiz coming up on today's Moment of Science.

You fill the ice tray up to the rim and put it in the freezer. The ice that results will be

a.) at the rim

b.) over the rim

c.) under the rim.

Answer? It depends.

I know, you hate that. But it does depend -- it depends on how long you wait before checking on the ice again. My guess would be that most folks chose answer b: over the rim. That is correct, if you checked on the ice trays after only a couple hours. That's because water expands when it turns into ice, as you know if you ever put a soda bottle in the freezer and came back to find it exploded all over the fish sticks.

But if you were to leave the ice tray untouched for a couple of months, you might find the ice to be under the rim, or lower than the water was.

Huh? Didn't we just say water expands when it freezes? Initially, yes. But frozen water can do something else as well: it can sublimate. To sublimate is to go from one state to another without passing through the usual intermediary state.

By adding heat, ice can be melted into liquid water, which can be evaporated. That's three states: solid, liquid, gas. But given enough time, ice can also turn into a gas without melting first! When your ice cubes seem to have shrunk, it's because they are sublimating: the frozen water is slowly evaporating off the cubes, without ever actually melting.

Will this work in any freezer? Next time.