There was a farmer who raised watermelons. He was doing pretty well, but he was plagued by local kids who would sneak into his patch at night and steal watermelons.
After some careful thought he comes up with a clever idea that he thinks will scare the kids away for sure. So he makes up a sign and posts it in the field. The sign says, “Warning, one of the watermelons in this field has been poisoned.”
The farmer goes to inspect his field the next morning and finds a new sign that says, “Warning, now two of the watermelons in this field have been poisoned.”