Laugh to Mute

Thing 2's arm is healing nicely.  She can go a couple hours longer than required for pain medication dosage, and has gotten outside quite a bit this week.
Today she and Thing 1 were working up ways to play on the swing which did not put her arm at risk.  They were  having a grand time, Thing 2 told me they "laughed until they went silent."  When they laugh and laugh but no sound comes out, just big gasps now and again.  Then she said, "That's something kids do when they think something is sooo funny!"  I told her I know what that is and she said, "It's great when you're having so much fun you laugh to mute!"