Microbes are everywhere—in your gut, on your skin, even in the sky. Along with water vapor and particles, clouds also contain living bacteria. And certain airborne species have a superpower: They can cause water to freeze into ice, which in turn prompts the heavens to open. Once frozen granules form around bits of dust, minerals, or tiny organisms, they fall, melting on the way down to become raindrops or forming flakes to make snow. This means the little life-forms might contribute to rainfall—but how much?