When we lift a pendulum, we transfer energy to it. This energy allows it to swing back and forth when we let it go. If we don't add any more energy, the ball will swing less and less until it finally stops. The arc of the pendulum will never be longer than the first arc when we let the ball go. It is this "conservation of energy" which means that the pendulum of doom will come close, but never actually hit you!
This activity is part of Science World's Big Science for Little Hands program. Activities were developed and tested with Preschool and Kindergarten educators.