A farmer with a goat, a cabbage, and a wolf is standing on the side of a river. They need to cross it, but there is only a small row boat which only fits two of them.
The goat is hungry, so if the farmer leaves the goat alone with the cabbage, the goat will eat the cabbage. The wolf is also hungry, and it will eat the goat if they are left alone.
How should the farmer proceed to get them all across the river?
Answer
The farmer and the goat rows to the other side. The farmer rows back and fetches the wolf. He brings the wolf to the other side, and then brings the goat back. He leaves the goat alone, and takes the cabbage to the wolf. Finally, he fetches the goat, and they are all on the other side.
Another answer to this puzzle: xkcd comic #1134.