Extreme sports (also called action sports, aggro sports, and adventure sports) is a popular term for certain activities perceived as having a high level of inherent danger.These activities often involve speed, height, a high level of physical exertion, and highly specialized gear. The definition of an extreme sport is not exact and the origin of the term is unclear, but it gained popularity in the 1990s when it was picked up by marketing companies to promote the X Games and the Extreme Sports Channel and Extreme.com launched. While use of the term "extreme sport" has spread far and wide to describe a multitude of different activities, exactly which sports are considered 'extreme' is debatable. There are however several characteristics common to most extreme sports. While not the exclusive domain of youth, extreme sports tend to have a younger-than-average target demographic. Extreme sports are rarely sanctioned by schools. Extreme sports tend to be more solitary than traditional sports. (Rafting and paintballing are notable exception, as they are done in teams.) In addition, beginning extreme athletes tend to work on their craft without the guidance of a coach (though some may hire a coach later). Activities categorized by media as extreme sports differ from traditional sports due to the higher number of inherently uncontrollable variables. These environmental variables are frequently weather and terrain related, including wind, snow, water and mountains. Because these natural phenomena cannot be controlled, they inevitably affect the outcome of the given activity or event. In a traditional sporting event, athletes compete against each other under controlled circumstances. While it is possible to create a controlled sporting event such as X Games, there are environmental variables that cannot be held constant for all athletes. Examples include changing snow conditions for snowboarders, rock and ice quality for climbers, and wave height and shape for surfers. Whilst traditional sporting judgment criteria may be adopted when assessing performance (distance, time, score, etc.), extreme sports performers are often evaluated on more subjective and aesthetic criteria. This results in a tendency to reject unified judging methods, with different sports employing their own ideals and indeed having the ability to evolve their assessment standards with new trends or developments in the sports.