A man is moving and to get to his new house he must cross a old bridge. He has a bag of grain, a dog, and a duck. He can only take 1 thing at a time. But if he leaves the dog with the duck the dog will eat the duck and if he leaves the duck with the grain the duck will eat the grain. How does he get all the things to the other side of the bridge?
answer: First he takes the duck. then the grain. On his way back he takes the duck, then the dog, the he takes the duck to the other side.