Picture a quiet gallery in the morning. A small painting is missing from its hook. Nothing is broken and nobody is hurt, but somebody was here who should not have been. How do you find out who, when all you have is a room?

Forensic science starts from one idea: a scene is a record. People and objects that touch each other swap tiny amounts of material, and those swaps stay behind. Reading the record carefully, and not jumping to a story, is the whole craft.