There is nothing quite like that moment in a mystery novel when the police break down a bolted door, only to find a corpse, a smoking gun, and - most importantly - no way for the killer to have escaped.

The Locked Room Murder (a subgenre of the “impossible crime”) is the ultimate armchair detective trope. It’s a magic trick in literary form. The author shows you an empty stage, performs a miracle, and then dares you to figure out the trapdoor.