The Market Trip
Riddle: You are going to a farmer's market. You have a wolf, a sheep, and a cabbage. You need to take them all across, but only one at a time. You can't leave the wolf with the sheep, for the wolf will eat the sheep. You can't leave the sheep with the cabbage, for the sheep will eat the cabbage. How do you get all three across safely?
Answer: You take the sheep across first. Then you go back, and take the cabbage across. Then, on the way back, you take the sheep back. Then you take the wolf across. Finally, you take the sheep back across, and you continue on to the market, all unharmed.
Source: https://www.riddles.com/8523
