You have an old-fashioned refrigerator with a small freezer compartment which could hold seven ice cube...

real-life/icecubes

Problem

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?

Show solution

You can make 48 cubes in the time it takes to freeze one tray (assuming
it does not take any longer to freeze multiple trays at once than it
takes to freeze one tray).  You can then continue to add 72 new cubes
for each increment of time it takes to freeze one tray.

Proceed as follows: fill one tray with water, put another tray upside
down on top of it, then another full tray, then an empty, etc.  This
gets you four trays or 48 cubes in the time it takes to freeze one
tray.

By using frozen cubes as spacers to hold the trays apart, you can make
an additional 72 cubes in the time it takes to freeze one tray.  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.