Why keys snap
A key blade thins at the shoulder over years of daily turns, and brass and nickel-silver blades fatigue the way a paper clip does when it is bent back and forth. A heavy ring hanging from the ignition adds side load on every bump, and using the key to pry open a box does the same damage in one go.
A stiff lock is the other half. On a cold morning in Julian or on Mount Laguna, or in a door lock nobody has used in years, the key meets resistance and the hand pushes harder. And a key that was already cracked, often visible as a hairline at the shoulder, needs only one ordinary turn to finish breaking.
What not to do with the piece still in the lock
Skip the super glue. It wicks past the broken blade into the wafers, and once it sets the lock is glued shut along with the key. Pliers and tweezers only grip the very end of the piece and tend to push it deeper as they slip. A spare key pushed in on top of the fragment jams it further and can snap the spare too.
If the piece sticks out far enough to pinch with your fingers, pull it straight out once, gently. If it does not come, stop there. A piece that sits flush with the face of the lock, or below it, is a job for a thin extractor.
Why the broken piece is worth keeping
The blade in your hand, laid next to the tip we pull out, still shows every cut. We read those depths and cut the new key to them, which is more accurate than guessing from a worn lock. The new key's chip is then programmed to the immobilizer at the car, so the blade is measured rather than guessed and only the chip is new.
When the pieces are too worn to read, the cuts come from the key code for your VIN instead. Either way, cutting and programming happen in the same visit.