Racers are generally categorized into separate professional and amateur categories. Amateurs, who make up the large majority of triathletes, are often referred to as "age groupers" since they are typically further classified by sex and age; which offers the opportunity to compete against others of one's own gender and age group. The age groups are defined in five- or ten-year intervals. There is typically a lower age limit; which can vary from race to race. In some triathlons, heavier amateur athletes may have the option to compete against others closer to their own weight since weight is often considered an impediment to speed. As an example, under USA Triathlon rules, "Clydesdale" athletes are those men over , while "Athena" athletes are women over . Other races and organizations can choose whether or not to offer Clydesdale- and Athena-type divisions and set their own weight standards. Depending on the type and size of the race, there may be any of the following methods implemented to start the race. In a mass start, all athletes enter the water and begin the competition following a single start signal. In wave start events, smaller groups of athletes begin the race every few minutes. An athlete's wave is usually determined either by age group or by predicted swim time. Wave starts are more common in shorter races where a large number of amateur athletes are competing. Another option is individual time trial starts, where athletes enter the water one at a time, a few seconds apart. The swim leg can occur in any available body of water, whether a natural body of water or a swimming pool. Swim legs not occurring in a swimming pool are considered open water swimming where the course typically proceeds around a series of marked buoys before athletes exit the water near the transition area. Racers exit out of the water, enter the transition area, and change from their swim gear and into their cycling gear. Competition and pressure for faster times have led to the development of specialized triathlon clothing that is adequate for both swimming and cycling, allowing many racers to have a transition that consists of only removing their wetsuit, cap, and goggles and pulling on a helmet and cycling shoes. In some cases, racers leave their cycling shoes attached to their bicycle pedals and slip their feet into them while riding. Some triathletes don't wear socks,