A bicycle is one form of transportation that can actually be stored inside your home. As the word implies, a standard bicycle has two wheels. Most bicycles (bike for short) have a frame to hold the 2 wheels, handles for steering, pedals, a seat and brakes. Bikes can also have gears, baskets or panniers for carrying things, a water bottle holder, lights, reflectors, a bell, an engine (some capture energy on the downhill that can then be used to power later parts of the trip or light).
Although a bicycle may have an engine or generator that can use a battery or store some energy to run lights or help on tougher parts of the ride, for the most part bicycles are "human powered" and energy efficient.
- Wikipedia's article on bicycle