The farmer came to the bank of a river and rented a boat. But crossing the river by boat, the farmer could carry only himself and a single one of his purchases: the wolf, the goat, or the cabbage. If left unattended together, the wolf would eat the goat, or the goat would eat the cabbage.
How will they cross?
Take the goat over. Return and take the wolf (or the cabbage) over and bringing the goat back. Now he can take the cabbage (or the wolf) over, and finally return to fetch the goat.