The ceilings are so high, the hollow noise of their skateboard wheels rolling over the wooden ramps, like the hum of a bowling ball gliding down a lane, sends sound waves skyward until they're gone. Many younger riders tend to define the sport of skateboarding differently because they have grown up during a time when skating has been promoted as an "extreme sport," one in which a skater's expertise is measured by the complexity of the street "tricks" he can pull-off.