Various versions of this joke exist, this one via Extreme Bartending: Bar Jokes.
SHERLOCK HOLMES AND THE MISSING LEMON
One day Sherlock Holmes and Watson sat down to enjoy their favourite lunch of fish and chips. Just as they were about to tuck in, Holmes suddenly stood up and shouted.
"Watson stand up!"
Confused, Watson stood up.
"Watson! Drop your trousers!" he barked.
Worried, Watson loosened his belt and dropped his trousers.
"Watson! Bend over!" Holmes ordered.
Watson bent over.
Holmes then plucked the wedge of lemon from Watson's plate and shoved it straight up Watson's arse.
Watson screamed "In God's name Holmes, what on earth are you doing?"
To which Holmes calmly replied "A lemon-entry my dear Watson ... a lemon-entry ..."
Holmes & watson, sexual