3 Old Ladies

3 old ladies are eating dinner, 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." "You think that's bad?" The second lady replied. "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, known on wood." She taps the table. With a startled look on her face, she asks, "Who's there?!"