What is the one device/gadget that you can keep happily using for the rest of your life?

If you have the option to keep only one of your devices/gadgets with you for the rest of your life, which will you choose? Like car, watch, mobile...etc. You will be stuck with it and cannot get anything new.

Why would you keep it? Longevity? Usefulness?

Rules and assumptions:

1) One device only, irrespective of functionality/purpose

2) You have access to power and basic support

3) It should be something you already own