In my neighbourhood lives a man who weighs 200 pounds. He has two sons. They both weigh 100 pounds each. On a festival day they decide to go across the river on a boat to visit some relations. But the boat could carry a maximum load of only 200 pounds.
Yet they managed to get across the river by boat. How did they?
First the two sons rowed across the river and one stayed behind while the other returned in the boat to his father.
The son remained behind while the father crossed the river. Then the other son brought back the boat and the two brothers rowed over together.