Crossing the River of Jealousy
There are three men, young, handsome, and gallant, who have three beautiful ladies for wives; all are jealous, as well the husbands of the wives as the wives of the husbands. They find on the bank of a river over which they have to pass a small boat which can hold no more than two persons. How can they pass so as to give no rise to jealousy?
Source: Leonardo Fibonacci, Italy, 1200