1/20    I need to demonstrate a special case of polymophism.  Say I have
        a "class A" and a "class B"  B inherits from A.  A defines foo
        and bar. B redefines bar.  A.foo() calls this.bar(), so if we
        call b.foo() we expect foo() to call the correct bar (B's).
        Can anyone think of a simple real world case where this happens?
        It's for a short talk I need to give, and I need a good example.