(A C++ example, that is)

	// "Toy" is an abstract base class.
	class Toy {
            public: virtual void playWith() = 0; // Note the shiny "= 0".
	};

	// "Cap''''''Gun" is one of the concrete classes.
	class Cap''''''Gun: public Toy {
            public: virtual void playWith() { /* Pow! */ }
	};

	// "Football" is the other concrete class.
	class Football: public Toy {
            public: virtual void playWith() { /* Punt! */ }
	};

	// And "Snot''''''Nose''''''Kid::enjoy(Toy&)" is a PolymorphicFunction
	class Snot''''''Nose''''''Kid {
            // Note how the type used is that of the base class.
            public: void enjoy(Toy& t) { 
                t.playWith(); 
            }
	};

	void f() {
		CapGun	gun;
		Football ball;

		Snot''''''Nose''''''Kid kid;

		kid.enjoy(gun);
		kid.enjoy(ball);
	}

The point here is that a "Toy" is just a concept, a certain kind of thingy, not
a tangible thingy. (Or, in OOP terms, "Toy" is purely abstract, a class of
objects, not a concrete object.)

There is no reasonable implementation for Toy::playWith().  How do you
"play" with a toy that you know nothing about?  You can look at it... fidget
with it... but in the end the way you use a Cap''''''Gun is nothing like the way
you use a Football, even though they are both Toys.

Observe:

	void g()
	{
		Toy toy; // Compile-time error!  You can't instantiate a Toy
			 // directly.

		Snot''''''Nose''''''Kid kid;

		// This kid doesn't know how to play with such a generic toy,
		// so it's a good thing the compiler didn't let him become 
		// frustrated.
		kid.enjoy(toy);
	}

Why not just use some generic mechanism, such as casting or templates?
Well, consider the following code:

	// Note that this doesn't inherit from "Toy".
	class Real''''''Gun {
	public:
		void
		playWith() { /* Yikes! */ }
	};

	void h()
	{
		Real''''''Gun gun;

		Snot''''''Nose''''''Kid kid;

		kid.enjoy(gun); // Compile-time error!  Kid's shouldn't 
				// play with Real Guns!
	}

So, remember kids:  Every gun is a loaded gun, and tell an adult if you find
a switch statement.