After wandering around lost in the desert two travelers come across a magic refrigerator. Inside, they find a jug full to the rim of eight cups of ice cold water. Additionally, there are two empty jugs, one which can hold exactly three cups, and one which can hold exactly five cups.
The travelers, although very thirsty, realize that it is important to divide the water up fairly. How can they do so using only the three jugs?
Although this puzzle is quite old, I first came across it on Cut the Knot.