One evening, Dr. Watson paid an unexpected call on Holmes.
"Is he expecting you?" asked the housekeeper.
"No," said Watson, "but I just need to speak with him for a minute."
"I don't know what he's up to," said the housekeeper, "but he left very strict instructions not to be disturbed until nine o'clock".
"I'll wait downstairs in the library," replied Watson.
A few minutes later, Watson heard the unmistakable sound of girlish laughter coming from the detective's bedroom, followed by shrieks of excitement from Holmes.
As nine o'clock approached, Watson could hardly contain his curiosity. Finally, Holmes came down the stairs, accompanied by a pretty dark-haired young girl in a school blazer and plaid skirt.
As soon as she left, the good doctor cried out, "Holmes, just what kind of schoolgirl was that?"
"Elementary, my dear Watson."