You need a manager, guard, three workmen, and a few large cardboard boxes. Setting: a factory gate.
Manager (to new guard): I'm giving you the very responsible position of gate guard at this factory. Because of the lack of vigilance by your predecessors, the workers have stolen so many finished articles that the firm is heading for bankruptcy. Your duty is to ensure this is brought to an end. Do you understand?
Guard: Yes sir. I am to stop stealing.
Manager: That's right. You can search people if necessary. Now it's up to you, and let's see some results.
Guard: Very good, sir. (Manager leaves; guard takes post; first workman enters carrying a cloth-draped box.)
Guard: Just a moment. What have you got in that box?
Workman 1: What do you mean?
Guard: What have you got in that box? It's my duty to see that no one takes stuff out of the factory.
Workman 1: Why didn't you say? There's nothing in the box. Look! (He shows everyone the box is empty).
Guard: Oh, well, that's all right then. (Workman 1 leaves and second workman enters, box draped as before. Guard and workman go through routine of looking in box. Repeat with the third workman. After the third man has left, manager races in enraged).
Manager: You idiot. I hired you to stop this pilfering. You've only been here half an hour and already we're losing things!
Guard: But the only people who went out were three men with boxes. I stopped them all and they had nothing in them.
Manager: You fool! We make boxes!