When the rocks outside get too hot to touch, climbers have two choices: Head north or head indoors. For the latter option, you can't do better than the Phoenix Rock Gym (which is actually located in Tempe, near Arizona State University). The place has a good mix of walls for beginners and advanced climbers, and a good mix of people, too. On the same afternoon you can see young couples introducing their 5-year-old to climbing and hear muscle-packed experts chat about drop-knee technique. The top-rope walls become crowded on Tuesday and Wednesday evenings, but that's the price of success. Thrill-seekers can always move into the lead wall area, where climbers practice life on the "sharp end" and sometimes fall up to 10 to 15 feet on a rope before being caught by a belayer. One of the original rock gyms in Phoenix, if not the first, the PRG recently went through a major renovation in which the owners added a second bouldering area upstairs, this one with more heavily inverted walls. If you want to see human spiders climbing upside down, this is the place. The "old" bouldering area (for the uninitiated, bouldering is the art of traversing relatively low walls without being tethered) received a summer makeover this year, and now sports murals of colorful cartoon sea creatures. If they were to add bunks and a vending machine with beer, we'd probably move in.