The king had a beautiful princess. She fell in love with a poor boy in the city, but the king strongly opposed. The princess insisted on her feelings, and finally the king made a request, as long as the poor boy could do it, he agreed to the two to marry.

The king asked the poor boy to take a boat to transport a wolf, a sheep and a basket of grass to the other side of the river. However, the boat the king gave him can hold a person, a wolf, a sheep, or a person, a sheep, or a person, and a basket of grass. Everyone knows very well that if no one is guarding, sheep will be eaten by wolves, and sheep will eat grass.

The poor boy stood on the bank, thought for a while, and then began to cross the river. Unexpectedly, he went back and forth several times and actually transported all three things to the other side of the river.

What method does this poor boy use?

[Reference answer]

First transport the sheep to the other side of the river, then the poor boy returns to the bank alone; then take the grass across the river, lay down the grass, and bring the sheep back; then put the sheep down, cross the river with the wolf, and the poor boy back again On the shore, finally, take the sheep across the river.