A physicist, a mathematician and an engineer are arrested and are to be executed by the electric chair

First, they strap the physicist and pull the lever and nothing happens. Thinking its a sign from God, the physicist is set free. Then, they strap in the mathematician and pull the lever and again, nothing happens. Thinking its a sign from God, they release the mathematician. Finally, they bring in the engineer and strap him in. As they were going to pull the lever, the engineer suddenly exclaimed, "Wait! The chair is not plugged in!"