In Money Heist S2E1, the lady inspector's husband collects a piece of a photo burned by the professor.
How can the police determine the professor's identity on the basis of that tiny piece of burned photograph?
Earlier Professor was burning old photos of his father in the furnace, as well as a newspapers clipping of how his father was shot dead in a bank robber and there could be connection to him. Also he had no idea how bad/good photos it could be, so Professor rather replaces it with fakes.
Police could for example search all possible newspapers and compare it with the piece of paper. With the right software they could do it really fast.