Acting is such an over-crowded profession that the only advice that should be given to a young person thinking of going on the stage is "Don’t" ! But it is useless to try to discourage someone who feels that he must act, although the chances of his becoming famous are slim. The normal way to begin is to go to a drama school. Then the young actor or actress takes up work with a theatrical company, usually as an assistant stage manager. This means doing everything that there is to do in the theatre: painting scenery, publicity (宣传 ), taking care of the costumes (舞台服装), and even acting in very small parts. It is very hard work indeed. The hours are long and the salary is tiny.
Of course, some people have remarkable chances which lead to fame and success without this long and hard training. Connie Pratt, for example, was just an ordinary girl working in a bicycle factory. A film producer happened to catch sight of her one morning waiting at a bus stop, as he drove past in his car. He stopped and asked if she would like to go to the film studio to do a test, and she thought he was joking. It took the producer twenty minutes to convince Connie that he was serious. The test was successful. And within a few weeks she was playing the leading part opposite one of the most famous actors of the day. But chances like this happen once in a blue moon!
The concluding sentence "chances like this happen once in a blue moon" means ______.