I'm sitting in my car on a flat road while it is stationary. The only forces acting on my car for this exercise are gravity and the normal force, which add up to give no net external force on my vehicle and me. My center of mass is at a fixed point somewhere in the vehicle, and according to the principles of Newton's laws, this fixed point should not move unless an external force outside of the car acts on the vehicle. If I was to raise my right arm, or do so little as to move my finger, my center of mass surely enough has changed its position, however no external forces have taken any effect. How is this explained?