People always feel sorry for the marine life caught in the aquarium. The square glass tank is like a cage to imprison them, but the ocean is just an irregularly shaped cage.
Apart from oceans, plains, mountains, and deserts are all cages, and the earth is a huge cage composed of these cages.
In this cage, oxygen, sunlight, rain, clouds, breeze, heavy snow, plants, animals, and tiny bacteria, seem to be carefully designed and arranged on the surface of the earth. Outside the earth, there is a silent universe - all sounds on the earth are separated by clouds, and joy, anger, wailing, and sadness only happen in the cage, just like human beings cannot hear the voices of sea creatures inside the glass.