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

Judges love to keep a straight face.

法官喜欢保持一张正直的脸。

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.

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