Identification

You misplace your expensive watch at the store and only find out the next day. So you go back to the store and ask customer service if they found it.

The temptation is to celebrate and give the thing back. It feels good to give something back to their rightful owner.

But if they do have the watch, the clerk faces a decision. They need to make sure that that's who you are.

They need to identify you.

The watch has qualities, like color, size, or branding. They found it at a specific place, at a specific time.

If you are who you say you are, you should know about certain things. You hold information that few others hold.

They hold a copy of the same information. But their copy is secret.

To succeed, they need to ask you the right questions. And they have to do it while not revealing their secret information.

You're good at inference. One small detail can give a secret away.