Mother, father and two children have to cross the river. They find a boat nearby, but it is small and can accommodate one adult or two children at a time. They are all good rowers, but how can the whole family get to the other side of the river?
First, the siblings get into the boat and cross the river. One of the children stays and the other returns to their parents. Then the child gets out and the father gets in. The father swims to the other bank and exchanges with the child waiting there. The child swims to the other side. Then the siblings get on again and cross over to the other shore, one child stays and the other returns. The mother gets in, crosses to the other shore and exchanges with the child. Then the child returns to the other bank and the siblings cross the river together.
