Three little old ladies

Three little old ladies are sitting in a restaurant one day, talking about this and that. The first lady said, "You know, I'm really getting forgetful. This morning I was standing at the bottom of the stairs and I couldn't remember if I was just about to go up or if I had just come down." "Oh, that's nothing," the second lady said. "The other day I was sitting on the edge of my bed, wondering if I was going to bed or if I had just gotten up." The third lady smiled pleasantly at the other two. "Well, my memory is just as good as ever, knock on wood." She rapped on the table with her knuckles, then gave a start and said, "Who's there?"