Barry Worked At A Coal Mine

One day he was leaving work with a wheelbarrow which had a box on it. The guard, looking at him suspiciously, stopped him and asked, "What's inside that box?"
Barry: "Nothing"
The guard opened the box, saw it was empty and let Barry go.
The next day the same thing happened. This kept going on for about a month, until some day when the guard stopped Barry and said, "Look, Barry. I know you're up to something. I think you're stealing something, but I don't know what. Just tell me what it is that you're doing, and I promise to let you go."
Barry: "Every day I steal a new wheelbarrow. I use the box as a disguise."