What does the idiom cracked a grin mean?

"Cracked a grin" is an idiom that means to smile slightly or briefly. It suggests a smile that is not a full-blown, beaming smile but rather a subtle, almost involuntary expression of amusement or pleasure.

Here are some examples of how it can be used:

* "He cracked a grin when he saw the surprised look on her face."

* "The comedian cracked a grin after delivering his punchline."

* "She cracked a grin at the thought of the upcoming vacation."

The idiom emphasizes the suddenness and lightness of the smile, implying that it wasn't planned or forced but rather a natural response to something amusing or enjoyable.