There was a farmer who grew watermelons in his field. Unfortunately, his farm was close to a main road and he had constant problems with people stealing his watermelons during the night. He was determined to stop this and came up with a cunning idea.

In the middle of his watermelon patch, he placed a sign that read, "Warning! One of these watermelons has been injected with poison!"

The next day, the farmer came out to survey his field and was delighted to find that no watermelons had gone missing during the night. However, he noticed that at the bottom of his sign, someone had written, "Now there are two!"