You have a dog, a cock and a sack of grain. You want to cross a pond, but you can only carry one at a time. If you leave the dog with the cock, the dog will eat the cock; if you leave the cock with the grain the cock will eat the grains.
How can you get all three across safely?
1st cross with Cock, next with grain, then bring Cock back and cross with the Dog and at last come back and take the Cock.