English

I heard "have a good one!" twice a day two days straight.
Since these were all after checkout at a car dealer, a hospital, and supermarkets, the situation explained me the meaning of it. It's just a simple or intimate way of saying "Have a good day!"
But I first wondered thinking "what should I have??"

Another unfamiliar expression was "say when!" which a friend of mine taught me.
This can be used at a restaurant when the waiter/waitress serves pepper or cheese etc. We tend to answer "OK", "good" or "thank you" when it's enough, but "when!" can be the answer instead of saying "thank you". I encountered this situation twice or three times at a restaurant for real.

These are simple and often used, but we don't learn them at school in Japan.
Discovering valuable expressions like these stimulates my desire to be natural in speaking :)