Reuven smells a certain food, and it makes his mouth water. "That food smells so delicious," he comments. "Such a pity it's prohibited to eat it!"
Reuven's Rabbi overhears his comment, and says, "Reuven, if you had thought the food was permitted to eat, you would not have been allowed to eat it. But since you thought it was prohibited, you may eat it right now."