Imagine you are a left-hander. Suddenly everything in the world is changed. The door locks are on the wrong side of doors. The shift gear (变速器) in your car is in the wrong place. Handles on tin openers are on the wrong side and turn the wrong way. Twenty-five million Americans wake up every day in just such a sorry situation. They are one in ten of us who are left-handed and must face the world designed for the right-handed majority. Why we are left or right-handed remains one of the greatest unsolved mysteries of science. We know that nearly two out of three lefties are men and that left-handedness runs in families. According to one study almost half of the children of two left-handed parents will be left-handed. The Scots-Irish family Kerr produced so many left-handers that in 1470 the family built its house stairway with a reverse twist. On the other hand, heredity (遗传) alone cannot explain lefties. At least 84 percent of them are born of two right-handed parents. And in 12 percent of twins, one will be right-handed, the other left. Perhaps the greatest puzzle of all is not why some people are left-handed, but rather why so few are. In fact, almost equal numbers of individuals will favour either the right or the left. However, scientists are trying to set things right and they are beginning to study the many ways left-handers differ from right-handers by considering how their brains work. How many of the Americans are left-handers?