Mrs. G. knows that a dumb joke is coming. Probably one she has heard before. Often. Nonetheless, she gamely counters, "Who's there?"
I have to digress here a moment. This is, indeed, one of my favorites. The knocker says "Interrupting starfish." Before the knockee can finish saying "Interrupting starfish who?" the knocker grabs the knockee's face with one spread, five-fingered hand. Hence the starfish. Also the interruption.
So I got as far as "Interrupting Starf-" when my wife, realizing which dumb joke she would have to put up with, put her face in her hand. And then realized that she had completed the joke for me. Self-interrupting starfish!
She suffers much for art.