The Key That Fits — How does a key open a lock?

A key opens a lock by pushing several small pins to exactly the same line at exactly the same moment, and the bumps cut into the key are the pin lengths inside the lock, upside down. In this lesson a brass door lock lies open on a bench with the key that snapped in it beside it, and you pick it with your thumb one pin at a time before filing a blank key until every pin drops onto the line and the lock opens in a single push. Then you fix a copy that has never turned in its owner's door, with the mechanism hidden behind a brass plate, by reading which pin stops the plug and cutting that one tooth a step deeper. It ends where every lock lesson should end: with your own front-door key in your hand, and the pins inside your own lock readable off its bumps.