Old Ladies' Noggins

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 sleep or had just woken up!

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