KentuckyFC writes "Water is the most abundant solid material in space. But although astronomers see it on planets, moons, in comets and in interstellar clouds, nobody has been able to show how it forms. In theory, it should form easily when oxygen and atomic hydrogen meet. The problem is that there is not enough of it floating around as gas in interstellar dust clouds. So instead, the thinking is that water must form when atomic hydrogen interacts with frozen solid oxygen on the surface of dust grains in these clouds. Now Japanese astronomers have demonstrated this process for the first time in the lab in conditions that simulate interstellar space. That's cool because all the water in the solar system, including almost every drop you drink on Earth today, must have formed in exactly this way more than 5 billion years ago in a pre-solar dustcloud (abstract)."