Knowing about yourself means not only that you find out what you're good at and what you like, it also means discovering what you're not good at and what you don't like. They both help you to see your aims(目标) in life.
Although most students would be unhappy if they found that they had failed a very difficult physics course，they have in fact learned a lot about themselves. They know that they should not become engineers or physical scientists. So failing can help a student to live a much happier life if he or she draws something out of the failing. They may then decide on their aims and choose the kind of work they would be fit for and they would like to do.
It is impossible to decide whether or not you like something until you have tried it. If you decide that you would like to play the piano，you need to take more than one lesson before you can know whether you are really interested in it or you are able to do it well.
It is not enough to want to be a great pianist. You also have to like the hard practice and long training before you become one. If you would enjoy being a great pianist but hate the work, forget it.
It's a good plan to try as many ideas as possible when you are still young. And then you will be able to find out what your true interests are.
