Defined Key Commands are Stuck At one point, in my misguided youth, I set my left "fn" key to do something silly. "No worries, surely this can be quickly undone", I thought. But now I can't get it undone. I've gone to system preferences -> keyboard -> shortcuts (which is where (I think) I made the changes in the first place) and asked for a restore to defaults. But, alas, the binding remains, surprisingly and frustratingly. I'm looking for (preferably non-drastic) further things to try.

EDIT: Silly me. 

(1) There's only the one "Fn" key. 

(2) My current machine is an early 2015 MacBook Air. Oddly this error has persisted as I've changed machines over the last several years. Every time I get a new machine I have them sync up the two (or whatever you call it) at the Mac store, and every time this error comes along for the ride.

FURTHER EDIT:

Apparently it matters to some people what the particular silly thing is that I once set my "fn" key to do. I'm not sure why, but if it helps, here it is: when I hit "fn" I go to the "mission control" screen. 

So.. yup, that's what happens. I want it to unhappen. But "restore to defaults" isn't doing the trick...
If you prefer the top row of keys to always behave as standard function keys without holding the Fn key:


Choose System Preferences from the Apple menu.
From the View menu, choose Keyboard.
Click the Keyboard tab if it is not already selected.
Enable "Use all F1, F2, etc. keys as standard function keys."