maintain sufficient steam pressure for long periods and was of little practical use. 1817 Push bikes, draisines or hobby horses were the first human means of transport to make use of the two-wheeler principle, the draisienne (or Laufmaschine, "running machine"), invented by the German Baron Karl von Drais, is regarded as the forerunner of the modern bicycle (and motorcycle). It was introduced by Drais to the public in Mannheim in summer 1817. 1885 Karl Benz built (and subsequently patented) the first automobile, powered by his own four-stroke cycle gasoline engine in Mannheim, Germany 1885 Otto Lilienthal began experimental gliding and achieved the first sustained, controlled, reproducible flights. 1903 Wright brothers flew the first controlled, powered aircraft 1907 First helicopters Gyroplane no.1 (tethered) and Cornu helicopter (free flight) 1928 Opel RAK.1 rocket car 1929 Opel RAK.1 rocket glider 1961 Vostok vehicle carried the first human, Yuri Gagarin, into space 1969 Apollo Program first crewed vehicle landed on the moon 2010 The number of road motor vehicles in operation worldwide surpassed the 1 billion mark – roughly one for every seven people. Types of vehicles There are over 1 billion bicycles in use worldwide. In 2002 there were an estimated 590 million cars and 205 million motorcycles in service in the world. At least 500 million Chinese Flying Pigeon bicycles have been made, more than any other single model of vehicle. The most-produced model of motor vehicle is the Honda Super Cub motorcycle, having passed 60 million units in 2008. The most-produced car model is the Toyota Corolla, with at least 35 million made by 2010. The most common fixed-wing airplane is the Cessna 172, with about 44,000 having been made as of 2017. The Soviet Mil Mi-8, at 17,000, is the most-produced helicopter. The top commercial jet airliner is the Boeing 737, at about 10,000 in 2018. At around 14,000 for both, the most produced trams are the KTM-5 and Tatra T3. The most common trolleybus is ZiU-9. Locomotion Locomotion consists of a means that allows displacement with little opposition, a power source to provide the required kinetic energy and a means to control the motion, such as a brake and steering system. By far, most vehicles use wheels which employ the principle of rolling to enable displacement with very little rolling friction. Energy source It is essential that a vehicle have a source of energy to drive it. Energy can