From Reader's Digest
April, 1982
Helen Bagby

     I play bridge regularly with seven other women, most of whom are seventy or older. Recently we celebrated the birthday of our oldest member by taking her out to lunch. When the waitress came to take our order, one of the women said to her, "This is a very special occasion. It's Elsie's ninety-second birthday."
     The waitress made seven instant enemies - and one fast friend - by asking, "...............

Answer:

"Which one is Elsie?"