A lawn mower has one or more rotating blades that cut the grass to a similar length. Other names for this tool include mower, grass cutter, and lawnmower. Even though the height of the cut grass may be fixed by the design of the mower, it is normally adjustable by the operator by means of a single master lever or a lever, nut, or bolt on each of the wheels. Blades can be powered by a corded or battery-operated electric motor, or they can be manually turned by pushing the mower forward, which causes them to spin because they are coupled to the wheels. Most self-contained lawn mowers are powered by internal combustion engines of relatively little displacement. Walk-behind mowers are self-propelled and just require a human to walk behind and control them; smaller mowers often have no propulsion at all and must be operated by a person to travel across a surface. Self-propelled walk-behind variants are the most common type of larger lawn mower, while ride-on types, which allow the operator to sit atop the mower, are also common. Lawn-mowing bots, as Automatic lawn mowers are more commonly referred to, can be set to operate completely independently or, less commonly, with the aid of a remote control.