And that reason is usually Physics. 

        Take flying for example; an object (like a plane) lifts off because of the dynamic of the air stream around it. The air stream around the plane is affected by the planes material, volume, weight, and movement. The plane's weight is variant upon the mass and acceleration forces. The plane's movement is determined through an examination of the forces acting as velocity, acceleration, friction, air pressure, weight (everything's tied together!) and potentially countless other things. 

        Despite everything we don't know, motion can be attributed down into three basic variables: Mass, Velocity, and Acceleration. From there, the most important forces that act on an object during flight may be determined. 

http://theiphonewalls.com/everything-happens-for-a-reason/

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