Cycling, the activity of riding pedal-driven vehicles like bicycles, unicycles, and tricycles, was popularized in Europe and North America during the late 19th century, and today over 50 percent of the global population knows how to ride. Bicycles have a fascinating military history, having been used for scouting and transport in conflicts from the Second Boer War to Vietnam, including Japan's deployment of 50,000 bicycle troops in its 1937 invasion of China, while Switzerland was the last country to disband its bicycle regiment in 2003. For everyday transport, utility bikes dominate in countries like Denmark and the Netherlands, which have the highest bicycles per capita, while specialized designs such as lightweight road bikes, rugged mountain bikes, and motorized e-bikes cater to different terrains and purposes. The cost of a new bike varies wildly, from around US$50 to over US$20,000—with the world's most expensive, a custom Damien Hirst Madone, selling for US$500,000—though UCI regulations require legal race bikes to weigh no less than 6.8 kg. When purchasing, experts recommend a test ride and careful consideration of the drivetrain, such as choosing between simpler hub gears or derailleurs, to best match the rider's skill level and intended terrain.