js里面的call的问题

Question

为什么这句superSay.call(this);就可以让浏览器先提示"Hello"，然后再"stu-Hello"呢function People(){}People.prototype.say = function(){    alert("Hello");}function Student(){}Student.prototype = new People();var superSay = Student.prototype.say;Student.prototype.say = function (){    superSay.call(this);    alert("stu-Hello");}var s = new Student();s.say();