(Photo: AP) The BMX scene started up in the '70s, gaining its first foothold among California teens seeking to mimic Motocross by using modified 20-inch bikes. Soon, dirt tracks and racing competitions began popping up around the world. Some of the most daring BMX riding takes place on halfpipes these days, with an array of tricks and feats that match those in skateboarding and snowboarding. Size Of Sport: 3.6 million Americans over the age of six tried BMX biking at least once in 2001, though that number represents a two-year decline of 1.7 percent. Most Excellent: BMX stunt rider Dave Mirra ties Tony Hawk for the most X Games medals ever won. Like Hawk he also stars in a popular video game. Talk And Tricks: Potato Chip: A wheel that is very bent Tailwhip: When the biker stands, grips the handlebars and rotates the bike frame 360 degrees around the axis. Brain Bucket: A helmet Sources: Sporting Goods Manufacturers Association, EXPN.com