The fan is the first component in a turbofan. The fan pulls air into the engine. The large spinning fan sucks in large quantities of air. It then, speeds the air up and splits it into two parts. One part continues through the "core" or center of the engine, where it is acted upon by the other engine components. The second part "bypasses" the Core of the engine, instead traveling through a Duct that surrounds the core to the back of the engine where it produces much of the force that propels the airplane forward.