There are four basic forces that work together to make an object fly. They act the same way on a bird, an airplane, a fling squirrel, or anything else that flies. These four forces are thrust, lift, gravity, and drag. Thrust is the force that moves something forward in the air. When something is moving forward, the air flowing over and under it creates lift. Lift is the force that keeps the object in the air. Gravity is the force that tries to pull all flying objects back to the land mass to which they came from. Gravity works against lift. When something is flying through the air, the air at which it is traveling through acts like an anchor and slows the object down. This force is called drag. If you have ever tried to run through water, this is the same feeling you get as the water runs against your legs. When one wants to design an airplane, there has to be balance between the lift and the thrust so that they will be stronger than gravity and the drag in the air. If the job has been done well, the plane will fly. If the person has not, it will most likely spin to the ground and crash.