You've got a sack of corn, a chicken, and a fox. You have to cross a river, but you can only carry them one at a time. If you leave the chicken with the sack of corn, the chicken will eat the corn. If you leave the fox alone with the chicken, the fox will eat the bird.
How can you get all three things across the river safely?
Cross the river with the chicken, then go
back and carry the corn
over the river. However, instead of leaving
with the bird, return together with the chicken. Cross the river for the third time with the fox, leaving
the chicken behind. Leave
the fox with the sack of corn
, and return for the last time to get the chicken. Done!