Scott Sandford, a senior astrochemist for NASA, points out that extreme pressure could also play a part in making snowballs. "If you squeeze water ice hard enough, it will melt," he explains. Packing snow into a ball could force some of it across that pressure threshold, making it liquefy and then refreeze. To get that kind of pressure, though, you'd need to be in space or a lab. In a casual snowball fight, surface moisture is still the most important factor. So what about the old theory that bare hands make the best snowballs? "It doesn't make any difference," Sandford says. "You're better off with gloves, so your hands don't get cold and numb."