Another Puzzle

A man and his two sons were walking down a road when they came to a river which had no bridge. The needed to cross the river, and they found a boat on the shore. Unfortunately, the boat could only carry 200 pounds. The father weighed 200 pounds, and each of his sons weighed 100 pounds. In spite of this, they all were able to cross the river using the boat without getting wet or sinking the boat. How did they do it?

Easy. Both sons crossed the river in the boat. One got out and the other returned to the other shore, where he got out. The father hopped in and rowed himself across. The son who had remained after the first crossing switched with Dad and rowed back and picked up his brother and hauled him back to the shore where they met up with the father. This puts all of them dryly across the river.

However, there was now no boat on the other side for the next trio that comes along!