I got a great "knock, knock" joke...

Hee hee, gotcha Allen, thanks for being the bravest one of the bunch, myself included.

You got here before I had a chance to explain the humor Igor, you are quick! This humor leaves the "Knocker" without a reply.

When the audience realizes it, they and the "Knocker" get the idea about the same time, the look of surprise and the laughter is contagious.

It is even better when the one who can trick the "Knocker" can look with anticipation for the punch line.