Just take his order, look him in the eye. If he says anything remotely funny, just laugh like you never heard anything so funny. And smile.
10 seconds sound clip from the Fast Times at Ridgemont High movie soundboard.
You can hear this line at 00:03:46 in the Blu-ray version of the movie.
Quote context
[...]
- Just made 16 bucks, Rats. What's the matter? You look depressed.
- I hate working the theater. All the action's on the other side of the mall.
- Yes.
- So, you working hard or hardly working?
- You look like you could still be in high school.
- I know. Everybody says that.
[...]