A transparent container (i.e. an old vase) and a coin with a string attached. The container sits on a table with a cloth over it. Have two guys start up a conversation where one tries to convince the other that the container and coin is a lie detector. The coin jumps up and down pulled by another person below the table when somebody tells the truth. The second guy doesn't believe the first guy that the vase is a lie detector.
Punch line is that the boy with the container and coin says he isn't lying and that he hasn't lied in his whole life where the table falls over with the container revealing the boy underneath.