Where does the "water in the sky" come from?

The one you mentioned seems to be a science and technology object in the science and technology museum. The answer to this question cannot but look at the three states of the atmosphere in the sky: solid, gaseous and liquid. I believe you should mean liquid. There is liquid water on the earth because the temperature and other conditions are suitable at the beginning of the earth. Venus has no liquid water because it is closer to the sun and the temperature is too high. In the early days of the earth's formation, various elements existed in a free state. Since then, due to the violent impact and the action of various rays, various simple substances formed by H and O have reacted violently with each other to form water.