18yr old in community college. Scared of unknown woman.

A girl pokes her key in the vending machine dollar slot. I casually say "I don't think keys go there" and smile. She looks up at me, slightly astonished, turns away and smiles faintly. Not knowing wtf to do, I say "just kidding..". What the fuck does the head turn and slight smile mean? Why didn't she respond?

Obviously "Just kidding.." was a mistake, but I had nothing else to say within 3 seconds. I could have added on, I.E. "Just kidding..but if you manage to unlock it can I grab some skittles?", reinforcing positive body language as well. Would the extra words have even been better, or does her silence say it all? How does one become wittier in such situations? Practice?

tl;dr PUA nub with minimal "skill"; anxious to approach but not to converse.