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();