A primer bulb (or bulb) is a small plastic bulb that is squeezed to force fuel into the carburetor of a small engine, which is then sucked into the cylinder by the vacuum created when the piston is at the bottom of the stroke. As the piston rises, the fuel mixes with air inside the top of the cylinder, creating an explosive mixture which the spark plug ignites, sending the piston back down to the bottom of the stroke. The primer bulb is one way of starting a chainsaw, compress the bulb until the engine turns over, which allows fuel to be sucked into the cylinder.