If you've gotten vaccinated recently, you might've noticed people (especially younger people) in the after care area swinging their arm around quickly, like a windmill. The move is a "hack" that went viral on TikTok as a way to minimize arm pain after the shot, and, according to Dr. Peter Chin-Hong, an infectious disease specialist and professor of medicine at UC San Francisco, it actually makes sense. Head over to SFGATE to learn some more.