Knock on Wood

Three old ladies are sitting in a diner, chatting about various things.

One lady says, "You know, I'm getting really forgetful. This morning, I was standing at the top of the stairs, and I couldn't remember whether I had just come up or was about to go down."

The second lady says, "You think that's bad? The other day, I was sitting on the edge of my bed, and I couldn't remember whether I was going to bed or had just woken up!"

The third lady smiles smugly. "Well, my memory's just as good as it's always been, knock on wood." She raps the table. With a startled look on her face, she asks, "Who's there?!"