THESE MICE ARE LAUGHING AT ME

Oddly, my newest batch of mice have never done this. And even the ones that did do it would be really sporadic about it; there'd be days where they'd just be going at it constantly, but then others where it'd be complete radio silence up in there.

When they first started, it was just one who'd do it, and then gradually the others would catch on and start at it too. Also, none from my current batch have ever done this. WHAT DOES IT MEAN!?