One day Frank was sitting halfway up the stairway when Susan came along and asked him what he was doing. He thought and thought and finally said, "I cannot remember if I was going upstairs or going downstairs."

Susan looked at him and said, "You old coot! Your memory is getting so bad. I will never get that forgetful." With that, she rapped on the stair railing, "Knock on wood!"

With that, she waited for a second, swivelled around and looked at the front door. "Who's there?"