A farmer wants to cross a river and take with him a lion, a horse, and a melon. There is a boat that can fit himself plus either the lion, the horse, or the melon. If the lion and the horse are alone on one shore, the lion will eat the horse. If the horse and the melon are alone on the shore, the horse will eat the melon. How can the farmer bring the lion, the horse, and the melon across the river?
Since it's not clear from the description: The boat will not cross the river without the farmer.
