system
Given the following post, respond as if you're commenting on Reddit:
human
Why is ice formed under slight pressure extremely brittle?

Ive experienced this with multiple ice cube trays in multiple freezers, that if you stack 2 ice cube trays to freeze, instead of making room to put them side by side, the top one works fine, the cubes pop right out, but the bottom one, the ice is really brittle, to the point that twisting the tray just results in a bunch of ice chips and a big mess. 

Does the slight pressure from the top tray interfere with the crystal formation of the ice in the bottom tray that much? Is there a name for this phenomenon, are there examples in nature?
gpt
It is not a matter of pressure, it is a matter of heat transfer.  An ice cube tray is designed to freeze the water from the top down, and being open on the top makes this the easiest place to remove heat from the water.  Also notice how the individual "cube" holders are slanted so they are smaller at the bottom.  As the top layer of water freezes, it pushes outward on the container and lifts itself up slightly.  The next layer down freezes, pushing the ice a little higher and this process continues all the way down.  What results is a strong cube that is relatively free from the sides of the tray and come out easily in one piece.

The tray on the bottom, however, cannot follow the same path since you have essentially insulated the top of the water.  Instead, the coldest area will be sides and bottom of the tray, and so freezing will happen on the surfaces of the tray first and move inward.  As this happens, the newly formed ice in the middle pushes outward on the older ice on the sides, creating a lot of stress and jamming the ice up against the side of the tray.  When you go to pop the ice out the twisting of the puts a lot of stress on the already stressed ice and it fractures.