crack a smile
crack a smile {v. phr.}, {informal}
To let a smile show on one's face; permit a smile to appear.
Bob told the whole silly story without even cracking a smile.
Scrooge was a gloomy man, who never cracked a smile.
When we gave the shy little boy an ice cream cone, he finally cracked a smile.
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Source: A Dictionary of American Idioms
