Meaning: put on a serious face to hide your real feelings.
Example: Maria said, “Well, that’s enough girls! You’ve been giggling all morning. I don’t want to ruin the surprise for Dad. So, please hide all the invitation cards before daddy comes home and don’t forget to keep a straight face while we are having dinner.”
Her daughters hugged her and did as they were told. They appreciated their Dad’s efforts and wanted to surprise him on his birthday by hosting a small party at home.
They hoped it would make him happy.