This article is from the Puzzles FAQ, by Chris Cole firstname.lastname@example.org and Matthew Daly email@example.com with numerous contributions by others.
You have an old-fashioned refrigerator with a small freezer compartment
which could hold seven ice cube trays stacked vertically, but there are
no shelves to separate the trays. You have an unlimited supply of trays,
each of which can make a dozen cubes, but if you stand one on top of
another before it's frozen, it will nest part way into it and you won't
get full cubes from the bottom tray. So, what is the fastest way to
make ice cubes?
By using frozen cubes as spacers to hold the trays apart, you can make
84 cubes in the time it takes to freeze two trays. Fill one tray,
freeze it and remove the cubes. Place two cubes in the opposite
corners of six trays, and fill the rest with water. Freeze all six,
plus a seventh you put on top, at the same time.