The loose skin of the upper arms which movement becomes apparent in certain high-wind conditions.
"I was driving with the windows down, and the sun was at an angle such that I observed a fluttering shadow out of the corner of my eye. I thought, 'my goodness! There's an animal in the car!' Y'know, like in Arizona, people worry about snakes getting into their cars. So I pulled over and...nothing; turns out the shadow was just my flying squirrels whippin' in the wind."