父类与子类,该怎么处理

class Person{    String name;    int age;    public void say()    {        System.out.println("I am fater");    }    public void work()    {        System.out.println("I hava a job");    }}class Child extends Person{    public void say()    {        System.out.println("I am child");    }    public void dance()    {System.out.print("I can dance");}}

    public static void main(String[] args) {        // TODO Auto-generated method stub        Person per=new Child();        per.say();        per.work();          Child ch=(Child)per;        ch.dance();        ch.say();        //为什么下面会报错呢？       Child ch1=(Child)new Person();    }