A boy is walking around in a carnival and he sees a single man in a booth. He walks up to the man and asks what the booth is for. "Well," says the man, "I can write your exact weight on this slip of paper. If I get it wrong, I pay you $20. If I get it right, you pay me $20." The boy looks around, and seeing no scales, agrees. 5 minutes later, the man has gotten $20 from the boy. How?
The man wrote "your exact weight" on a piece of paper.
I am used to make hydroelectric energy. What am I?