Whether they're rolling around slick city streets or climbing snow-packed mountain trails, fat-tire bikes are back in full force this winter season. For those who don't know, a fat bike is a mountain bike with 4- to 5-inch wide tires designed to handle snow or other slick surface (traditional cycling tires are about 2 inches). And with the growing popularity of the sport in Aspen — local cyclists and bike shop salespeople say they've watched the fat-tire bike trend become increasingly popular — and the fact that today is Global Fat Bike Day (check out http://www.fat-bike.com, if you don't believe us), we thought a spin around the snow with a few fat-bike aficionados would make for a perfect installment of A Week in the Life.