totally random shapes, or at least a large collection of nice looking but unrelated shapes.

you can see all the pieces in the collection ahead of time

-can you see the order you'll get them?
-can you pick them from the collection?

they're all made of ice
they slowly melt
there's a drain in the bottom of the floor
pieces keep coming, and if they reach the top (before enough melts and falls out the drain then you lose the game

maybe the bottom of the vat is heated.