Who will tell me a joke? I accepted with a smile.

A little girl went to buy apples. She saw a sign in front of an old man. It says: contains many scales. So she bought 5 Jin at the old man's stall. When she got home, she weighed it with a scale. Less than four and a half pounds. So she went back to the old man. Grandpa, it's not written on your sign. Do you pack many scales? The apple you gave me is half a catty short. The old man looked at the girl and said, little girl, you read it backwards. It should be read like this. Weighing is not enough.