A border crossing guard sees a well-known smuggler riding a bicycle with two heavy sacks on either side. He pulls him over and checks the sacks. There's nothing in them but sand. "Fine," the guard says, "you can go."
The next day he sees the same smuggler riding a bicycle with two heavy sacks. Again the sacks are filled with nothing but sand.
This goes on for weeks. Finally the guard says, "I know you're smuggling something. What is it?"