The greatest math blog ever.
A man is on his way to the local marketplace to sell a fox, a chicken and some grain. He has to cross a river, and his boat is just big enough to carry him and one of the other three.
When he arrives at the river, he knows he’s got a problem. If he leaves the fox and the chicken, the fox eats the chicken. Otherwise, if he leaves the chicken and the grain, the chicken eats the grain.
How does the man get the fox, the chicken and the grain to the other side of the river?