The ordinary lock on most front doors has a row of small pins standing across the seam between the part that turns and the part that doesn’t, and each stack of pins is cut in two at a different height. The right key lifts every stack until all the cuts sit exactly on that seam, and only then will it turn. Picking one works because no lock is made perfectly. The holes are drilled a hair out of line, so under a slight twist one pin jams before the others, and it can be nudged up until its cut reaches the seam and catches there; then the next one jams, and the next. A flawless lock would grip every pin at once and give nothing away. It is opened one small manufacturing mistake at a time.