+Robert Reynolds if that's a key or a lock ( which one?) then I don't see how it fits the riddle ( turn once and what's outside can't get in, turn again and what's inside can't get out)? Or am I just being to dense today, which is always a possibility.

+Tamas Ferencz I read the riddle the same way, and I’d heard it before; the answer had been ‘key, opening implement’, though I included ‘lock, closing implement’ as locks often have knobs of various sorts instead of keys. That said, I don’t fully understand, either. Perhaps “they” (the Authorities, if I recall The Hobbit’s scene correctly 🙂) mean turning a key in a regular lock (preventing outsiders from entering) and then again in a deadbolt (additionally preventing insiders from getting out without turning the key or lock-knob back again)? It feels like a stretch; perhaps we’re both just being dense today.

I indeed intended "key".

I turn around once,
What is out will not get in.
I turn around again.
What is in will not get out.

What am I?

+James Coish yes, that is clear. But in my head a lock is either open, in which case anyone can get through either way, or it's locked, and then no-one can get through from either side.

+Tamas Ferencz OK, I see your point. I thought maybe my translation was incorrect. I now agree its a stupid riddle. ☺