Basically, when a natural frequency acts on coke bottle, It resonates. During the resonance, the lump of air in neck of it goes under simple harmonic motion. It goes in and out the bottle, increasing and decreasing pressure of cavity of coke periodically. When air goes inside the bottle, the bottle does not move. However, when air flows out, it moves toward opposite direction of the opening. Why do you guys think this phenomenon happens? My teacher gave me two explanations. First one is about Action and Reaction. He related it to the directions pressure acts on and net force change that occurs when air goes in the cavity. Second one was about conservation of momentum. He said that conservation of momentum occurs when air flows out and same thing to "feynman sprinkler" occurs when air flows in. Which one do you think is more plausible?? Can you explain to me why you explain to me why such is the answer? To me, both explanations can be applied. Also, can momentum of lump of air be calculated? If so, HOW? Lastly, something not related to this, but is it the flux that affect strength of wind or velocity of fluid?