"Have you ever heard the old saying, "Never judge a book by its cover"? This is a good rule to follow when you try to judge the cleverness of others by their looks and performance. Some people have minds that shine only in certain situations. A young man with an unusual gift in novel writing may find himself speechless before a pretty girl. But don't make the mistake of thinking he might not be that clever. With a pen and paper, he can tell what he thought very well. Other people may fool you into mistakenly thinking they are clever by putting up a good front. A student who listens carefully and takes notes in class is sure to make a good impression on his teachers. But when it comes to exams, he may get scores much lower than his classmates. We may conclude that:you can't judge someone by appearances. The only way to tell whether a person is clever or not is to get to know him by observing  how he reacts to different situations. The more situations you observe, the more accurate your judgement is likely to be. So take your time. Don't judge the book by its cover. According to the passage, true knowledge of a person's cleverness comes from  _  .
the grades he gets in school his manners at parties his outward appearanc how he behaves in different situations