You have a fox, a chicken and a sack of grain. You must cross a river with only one of them at a time. If you leave the fox with the chicken he will eat it; if you leave the chicken with the grain he will eat it. How can you get all three across safely?
Take the chicken over and leave the fox and the grain. Go get the fox drop it off and pickup the chicken. Take the chicken back across and pickup the grain and leave the chicken. Drop the grain off with the fox and take the chicken over now all of them are on one side!