You’re down to your last key. You meant to get a new one cut and now’s there’s an emergency and you need to hand someone a spare.
This video shows how to make a spare padlock key. I am not recommending it for a door key, and haven’t tried it, so I’m not sure if a tin can lid would be strong enough for a door lock. But it’s a neat trick!
Remember, gloves, goggles, and all the safety stuff. Tin is sharp and can cut fingers. Tin snips apply pressure and you don’t want a piece of tin flying into your eyes. (Hospitals use powerful magnets to remove them, you know!) And hold the key with pliers when you burn it.