Afarmer is traveling with a fox, a sheep and a small sack of hay. he comes to a river with a small boat in it. the boat can only support the farmer and one other animal/item. if the farmer leaves the fox alone with the sheep, the fox will eat the sheep. and if the farmer leaves the sheep alone with the hay, the sheep will eat the hay.

how can the farmer get all three as well as himself safely across the river?