The thread title is meant to be "Can a fly lift a balloon", it's Tom now due to a small mistake while posting. This question was raised on a dutch technology forum. Specific: Can a fly in a balloon lift the balloon by flying upwards against the top of the balloon? More general: Can a flying object in a closed hollow object lift the closed hollow object if the flying object has enough power and the environment properties are the same inside and outside of the closed hollow object? Notes: - The closed hollow object is full with air exhaled by a human, not with hot air or helium. - Feel free to ask more specific details.