A ruler is pushed down into the closed fist. At the word "Go," the ruler jumps up to the ceiling.
Secret: Slip a rubber band over the second finger of the hand. Close the fist with the thumb in front and the elastic will not be seen. Push the ruler down into the fist so that it engages the rubber band, but clench the ruler firmly. As soon as you release pressure the ruler will jump out of the hand. The rubber band may be dropped on the floor.