Don't Judge a Book by Its Cover
(don't form an opinion about something based on appearance alone)
Have you seen the new guy they hired as coach of Commodore High's football team? He's so short and puny. He hardly looks like a football coach to me.
Listen, pal. Don't judge a book by its cover. From what I hear, that guy is one of the best. His size has nothing to do with his ability to produce a winning team.
How do you know that?
I was talking with one of the players on the team. He said that the new coach is a genius at strategies on the football field, and he puts the team through some of the roughest training they've ever had.
That surprises me. I guess I really shouldn't base my opinion about him on the way he looks.
puny = 弱々しい