What is a good analogy to use to explain to someone who is new to Bitcoin?

I attempted by trying to say that it's like a combination lock; where you first assume the lock is only usable if it's unlocked. You can pass the lock around, publicly, and nobody can use it, everyone can see it. Then, when it comes time to spend it (use it), it's only the owner of the key that knows the key, and it can be taken out of public (offline) unlocked, then put back into public and used (spent). In this example, the key is never at risk of being exposed in public.

Does someone have a better or cleaner analogy to use to explain to someone (ehhm, my 60 year old mother)?