This is my "hey, I've noticed that too!" question for the week. If you stack two plastic ice cube trays with water in them in a freezer, the resulting ice cubes in the top tray will usually come out cleanly and easily, while the those in the bottom tray will stick and crack. (Try it!) So... why?
Summarized answer, 2013-01-12
Ice expands as it freezes. So, if freezing starts at the top surface of the water, the liquid water below it allows the cube to edge gently upwards as it freezes, without much stress on it. This same upwards edging effect also keeps the ice from every gaining a solid hold on the entire surface of the tray. Result: The top surface of the ice cube stays flat, and the cube comes out of the tray easily without cracking.
Conversely, if freezing starts from below at the tray surface, the freezing water has the time and opportunity to lock itself tightly to all parts of that surface. But as the ice continues to grow inward, it tries to expand from all sides at once into the internal space. This creates a lot of stress in the ice, and causes the cube to form a peak at the top due to the internally expanding ice having no place else to go. So, since the ice has both had ample opportunity to freeze tightly to all of the tray surface, and because it was stressed internally as the freezing progressed, the ice cube becomes a disaster when you try to get it out. Much of it sticks to the tray, and the parts that do come out are often cracked.
How is all of this connected to whether a tray is stacked on top or bottom?
It's because stacked trays almost always impedes the cold air of the freezer from getting between the trays. That causes the stacked trays to behave largely like a single unit, with freezing occurring from the outside of that two-tray unit. The top tray therefore freezes mostly from the top down, producing ice cubes that have flat surfaces and come out easily. The bottom tray freezes mostly from the bottom up through the surface of the tray, and so ends up producing peaked ice cubes that stick to the tray and crack when you try to remove them.
(See the next addendum for some actual experiments you can do to try to confirm this.)
(Ron Maimon's was the first one to figure this out; see his accepted answer for details.)
Here's an experimental prediction based on Ron Maimon's answer:
If the ice cubes have flat tops, they will come out easily. Conversely:
If the ice cubes have central peaks, they will crack badly when you remove them.
See the answer summary above for why.
Based on the answer, this experiment should make ice cubes come out easier: Tape an empty tray to the bottom of the tray with water in it. This will create an air-tight insulating air pocket below the filled tray, and so should result in flat-topped ice that comes out of the top tray easily.
You can also try for he opposite result by taping a tray upside down over the top of the tray with water in it. When it is done freezing, the ice cubes should be strongly peaked and should stick and crack when you try to remove them.
So, anyone interested: Get crackin'!