The was a farmer who raised watermelons. He was doing pretty well but he was disturbed by some local kids who would sneak into his watermelon patch at night and eat watermelons. . .
After some careful thought he came up with a clever idea that he thought would scare the kids away for sure. So he made up a sign and posted it in the field. The next day the kids showed up and they saw his sign: "Warning, one of the watermelons in this field has been injected with cyanide."
Well the kids were bright and not about to risk another watermelon. They ran off, made up their own sign and posted it next to the sign that the farmer made.
The next day the farmer showed up to look over the field and he noticed to his delight that no watermelons are missing. He was perplexed, however, by a sign next to his. He drove his tractor up to the sign which read: "Now there are two!"