Seriously. The onboard APU's (Aux Power Units) are started first. These are small turbine engines @ 600 HP I believe. They are started with electric motors. Once they come up to speed the air volume and pressure outout they generate is ducted to the main engines to a device that looks much like a turbo on a car only bigger. The air flowing thru the turbo spins it up and it is coupled to the main shaft of the main engine causing it to rotate up to starting speed. Once up to speed, ignition and fuel are introduced and the fires are lit. The "start motor" turbine is then disengaged because it would disintegrate at full engine speed and the engine continues to accelerate to idle speed. The air from the APU is also used for Air Conditioning/Heat on the ground and generates electrical power as well. This is the way most modern commercial jets start these days. If the APU is broken a compressor truck can connect to the plane and provide this air.