A farmer, troubled by local children sneaking into his watermelon patch at night to eat his crop, devised a clever plan to scare them off. He posted a sign in the field that read: "Warning! One of the watermelons in this field has been injected with cyanide."
The next night, the children saw the sign and fled. However, they returned and posted their own sign next to the farmer's. When the farmer came to check his field, he found that no watermelons were missing. But next to his warning was a new sign that said: "Now there are two!"