Three ladies are trading old age war stories, but prepare for what the third says

Three ladies were discussing the travails of getting older. 

One said, "Sometimes I catch myself with a jar of mayonnaise in my hand, while standing in front of the refrigerator, and I can't remember whether I need to put it away, or start making a sandwich."

The second lady chimed in with, "Yes, sometimes I find myself on the landing of the stairs and can't remember whether I was on my way up or on my way down." 

The third one responded, "Well, ladies, I'm glad I don't have that problem. Knock on wood," as she rapped her knuckles on the table, and then suddenly said, "That must be the door, I'll get it!"