AMan invented a lot of sports equipment． The first true piece was the ball．Thousands of years ago，in Egypt，the children's favorite game was throwing stones． But a badly thrown stone could hurt a child． Looking for something less dangerous to throw，the peo-ple in Egypt made the first balls．At first，they used tree leaves to make balls． They used vines(藤) to keep the leaves to-gether． Later they began to use pieces of animals' skin and filled them with some light things．These balls were much better than the old ones．Before long they invented a lot of ball games，and each had its own rules． They began to play these ball games according to the rules． They thought that ball playing was one of the best ways to teach the young men to be brave and strong．( )21． The Egyptians invented _________very long ago．