Jack went to a barber's shop and had his hair cut, but when he came out, he was not happy with the result . When his friend Bob saw him, he laughed and said, "What has happened to your hair, Jack?" Jack said, "I tried a new barber's shop today, because I wasn't quite satisfied with my old one, but this one seems even worse." Bob agreed . "Yes, I think you're right, Jack. Now I'll tell you what to do when you go into a barber's shop next time: look at all the barber's hair, find out whose hair looks worst, and then go straight to him." "Why shall I go to him?" Jack asked. "But that would be foolish!" "Oh, no, it wouldn't," answered Bob. "Who cut that man's hair? Just think it. He couldn't cut it himself, could he? Another of the barbers cut it. So you know he can't be the worst barber." From the story we know that   _  .
it's foolish to have one's hair cut at a barber's shop barbers cut each other's hair barbers never have their hair cut a barber always cuts his hair by himself