from The Adventure of the Speckled Band
The device: inference from detail. A lady dies in a locked room; her sister hears a low whistle in the dark. Sherlock Holmes looks at a bed bolted to the floor, a bell-rope that rings no bell, and a ventilator that lets in no air - and reasons his way to the truth. He also admits where he first went wrong.
"I had come to an entirely erroneous conclusion, which shows, my dear Watson, how dangerous it always is to reason from insufficient data."
A detective story lets you play detective. Which detail told the most? And what could that detail NOT tell you on its own?