A bump key is a filed-down key that is used to open pin-tumbler locks. While locksmiths sometimes use bump keys to open locks that have been accidentally locked from the outside, criminals also use bump keys to break into homes, mailboxes, vehicles, and bike locks. Any cylinder lock is a target for a bump key and guarding yourself against this form of theft is important. Bump Key Proof Locks are on the market and are the only option we recommend. Bump keys work and work well.