java中多态中向下转型的问题

public class Father {
public void show (){
System.out.println("父类方法");
}
public void show1(){
System.out.println("父类二方法");
}
}
public class Son extends Father {
public void show(){
System.out.println("子类方法");
}
public void show2(){
System.out.println("子类2方法");
}
}
public static void main(String[] args) {
// TODO Auto-generated method stub
Father son=new Son();
son.show();
son.show1();
Son son1=(Son) son;
son1.show();
son1.show1();
son1.show2();
}

为什么son1.show1() 也会出现，该怎么理解？