Elementary my dear Watson

In fact the line doesn’t appear in the Conan Doyle books, only later in Sherlock Holmes’ films.

He does come rather close at a few of points. Holmes says “Elementary” in ‘The Crooked Man’, and “It was very superficial, my dear Watson, I assure you” in ‘The Cardboard Box’. He also says “Exactly, my dear Watson, in three different stories.

The phrase was first used by P. G. Wodehouse, in Psmith Journalist, 1915.