A professor's grandson asked his grandfather, "Grandpa, why did you say that all the false(假的) things are ugly"

"That's certainly true. Could you give me an opposite example"

"Oh, yes." the grandson, climbing on his grandfather's knee, said proudly, "Look at yourself, when you put on your false teeth, you look younger and lively; when you get them off, your mouth looks empty and shrivelled(枯萎).That's really ugly. Isn't it an opposite example" The professor could find no answer.