A man has a wolf, a goat, and a cabbage. He must cross a river with the two animals and the cabbage. There is a small rowing boat, in which he can take only one thing with him at a time. If, however, the wolf and the goat are left alone, the wolf will eat the goat. If the goat and the cabbage are left alone, the goat will eat the cabbage.
How can the man get across the river with the two animals and the cabbage?
This is a classic. I'm surprised it hasn't been asked before.
Take the goat. This leaves the wolf and the cabbage which is fine.
Take the cabbage. (To the goat.)
Return with the goat. So it can't eat the cabbage.
Take the wolf. So it can't eat the goat.
Return alone. Again leaving the wolf with the cabbage.
Take the goat.
Alternatively, you can swap the steps of taking the wolf and the cabbage.