I am a drop of water. I am pure and clean, and I come from heaven. I have been shaken

I am a drop of water. I am pure and clean, and I come from heaven. I have been shaken from the clouds, and have fallen on the earth, to refresh it, and to cleanse it. I am a servant of the stomach, and I digest the food which would otherwise be indigestible. I am a friend to the health of the body, and I am swallowed by the thirsty earth, in summer, and in winter I am frozen into a hard