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();
    }