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IT'S A STONE, LUIGI.
One peaceful day in Dinosaur World, Luigi presented a chisled stone to Mario. Stone Luigi claims it's a football that he chisled himself. Mario remarks "It's a stone, Luigi. YOU DIDN'T MAKE IT!"
Mario was told to have eaten those words as Luigi allegedly beamed the stone/football at his dear brother's abdominal area. Mario survived with 8 stitches in the pectoraland 5 broken ribs. The stone/football was "...official size, [but] not official weight".
Yoshi remained silent until he was given the chance to be on Mario's football team.
The scrap was not at all a loss, for Mario discovered that football would "...teach the [cavemen] organization and teamwork". We don't know why mario is so obsessed about organization and teamwork, but the last time the cavemen didn't meet these requirements, Mario threw a fit and was fined for 599 US dollars which, strangely enough, is what Chad Warden spent for a very special Playstation 3.
Custody of the stone/football is still underway.