A spare key is a strange thing to give someone. It does nothing on the day you hand it over, and most of them never do anything at all: they sit in a drawer across town for years, waiting for a locked-out evening or a burst pipe that may never come. So when you choose who gets it, you aren’t really choosing someone who will use it well. You’re choosing someone who won’t use it at all until you need them to. That is a narrower kind of trust than it looks. Plenty of people can be relied on to do something; a spare key asks for a person who can hold a way into your house for years and never once walk through it.