There was a farmer who specialized in watermelons. He was doing pretty well, but every so often, some of the local kids would sneak onto the farm and filch melons. So the farmer thinks about it and hits on a solution.
The next morning, he posts a sign that reads: "Caution: One of These Melons Has Been Treated With Cyanide."
The morning after, the farmer goes out, surveys the field, and nods with satisfaction. Every melon is still in place. His satisfaction curdles slightly when he goes to see the sign that saved his farm, and sees a smaller sign tacked up alongside it.
"Now there are two."