A farmer is on his way to market. He's going to sell a wolf, a goat, and a cabbage. On the way, he comes to a river crossing. The river is too wide to ford, so he has to take a tiny boat across. The boat is so small that he can take at most one of the three items with him in the boat at a time. Left alone, the wolf would devour the goat, and the goat would devour the cabbage. How can the farmer safely transport all three items across the river?
The wolf and the cabbage can be left together with no risk to either. So the farmer first takes the goat across. He returns to the near side, retrieves the cabbage, then takes it across to the far side. Then he returns to the near side with the goat. At the near side, he leaves the goat and takes the wolf to the far side, leaving the wolf with the cabbage. Finally, he returns to the near side to take the goat across for a second time. Problem solved!