A guy walked into a little corner store with a shotgun and demanded all of the cash from the cash drawer.
After the cashier put the cash in a bag, the robber saw a bottle of Scotch that he wanted behind the counter on the shelf.
He told the cashier to put it in the bag as well, but the cashier refused and said, "Because I don't believe you are over 21."
The robber said he was, but the clerk still refused to give it to him because she didn't believe him.
At this point, the robber took his driver's license out of his wallet and gave it to the clerk.
The clerk looked it over and agreed that the man was in fact over 21 and she put the Scotch in the bag.
The robber then ran from the store with his loot.
The cashier promptly called the police and gave the name and address of the robber that he got off the license.
They arrested the robber two hours later.
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Duh!