Poisoned Watermelons
Every morning the local farmer noticed someone had been in his watermelon patch during the night. Unable to stay up and catch them and unwilling to tolerate the constant loss, he put up a sign that said:
WARNING! ONE OF THESE WATERMELONS HAS BEEN INJECTED WITH A STRONG POISON
The next morning he went out to inspect his watermelon patch and found that his sign had been altered. It now said:
WARNING! TWO OF THESE WATERMELONS HAVE BEEN INJECTED WITH A STRONG POISON