pointing subclass object by superclass reference variable

pointing subclass object by superclass reference variable

when we can point the object of subclass with superclass reference
variable then why can't we access a subclass method with that reference
variable of superclass..
for eg. following code gives error..
class Child
{
void func()
{
System.out.println("abc");
}
}
class Parent extends Child
{
public static void main(String s[])
{
Child a=new Parent();
a.func();
}
}