Other forms: grinned; grinning; grins
When you grin, you smile. You're likely to grin when your sister tells you a particularly funny knock-knock joke.
There's some disagreement about the difference between a grin and a smile — some define a grin as a broad smile that exposes your teeth, but others consider a grin to be a smile with lips tightly closed. Either way, a grin usually expresses amusement, although it sometimes has a mischievous or sly implication: "No one confessed to letting the rabbit loose in the classroom, but one student's grin gave her away."