DUH II
From Reader's Digest

   At a supermarket, I put my items on the counter and handed the cashier a coupon that offered: "Buy three pounds of margarine, get one free." The clerk spent a lot of time scrutinizing the coupon.
"What's wrong?" I asked.
   "There are four pounds of margarine here," he said.
   "That's right."
   "But I have to ring them up."
   "All right. Do it."
   "But which one is the free one?"