Sierra Olea (R), 12, and Norelyn Herrera, 8, from Oxnard, warm up at the National Championship for Banked Track Roller Derby in Los Angeles, California, June 7, 2013. Roller derby is a real, full-contact sport, played at fast pace by all-volunteer teams on a banked oval track and was created in 1935 by Leo Seltzer in Chicago. Juniors are aged 7-16 and play on teams of ability, not age, and have feisty skater names such as Lil Bruiser, Ripper N Half, and Felony Melanie. Picture taken June 7, 2013. REUTERS/Lucy Nicholson (UNITED STATES - Tags: SOCIETY SPORT)