Grandma has an old-fashioned refrigerator with a very small freezer compartment which can hold up to seven ice cube trays stacked, but there are no shelves to separate the trays. You have fifteen trays, each of which can make a dozen cubes, but if you stand one tray on top of another before it's frozen, it will sink into the lower tray and you won't get full cubes from the lower tray. Without using anything but water and the ice cube trays, what is the fastest way to make full ice cubes in seven trays?
This can be accomplished by using frozen cubes as spacers to hold the trays apart. In the time it takes to freeze two trays, seven trays of ice can be frozen. 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 trays, plus the seventh tray which is on top.