What went through Tom's mind I can't imagine, but he did manage to keep a straight face.

汤姆心里在想什么我想像不出来，但是他确实装着一脸严肃。

《柯林斯英汉双解大词典》

We often keep a straight face or use an unsmiling expression when we're making a joke.

当我们开玩笑的时候，我们经常绷着脸或使用不笑的表情。

youdao

Just now, it is a struggle to keep a straight face when you read the words "talent" and "Wall Street" in the same sentence.

刚才，当你在同一个句子里读到“人才”和“华尔街”这些词的时候，会努力控制自己不要笑出声来。

youdao

"It tickles a bit when it's applied, " Ms. Pavenooch said, leaning back and closing her eyes as she tried to keep a straight face.

WSJ: In Thailand, Those Seeking A Golden Glow Rush to the Spa

It was then that he learned to keep his face straight.

ECONOMIST: M.R.D. Foot

At any hour, day or night, there is a Republican on TV complaining with a straight face that Al Gore will keep demanding recounts until he has enough votes for victory.

CNN: Why Gore Has the Right to Fight