Elementary my dear Watson
The supposed explanation that Sherlock Holmes gave to his assistant, Dr. Watson, when explaining deductions he had made.
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.