Hello. I hope you are having a good day.

Today's expression is: to keep a straight face.

It means: to not smile or laugh; to put a serious expression on your face when something very funny or amusing is happening.

Example 1: The comedian kept a straight face while he was telling his jokes, but everyone in the audience was laughing.

Example 2: I don't know how you can keep a straight face while you are telling me that story; it makes me laugh out loud every time I hear it. 

Enjoy the rest of your day!