On Earth, all life depends on water. Indeed, life originated in water, and the availability of water strongly influences the environmental distributions of different species. Furthermore, water is the single most abundant molecule in cells, so water is the medium in which the molecules of life interact. So important to life is water that, in the late 1990s, the National Aeronautic and Space Administration (NASA) announced that the search for extraterrestrial life would be based on a simple strategy: Follow the water. NASA’s logic makes sense because, within our own solar system, Earth stands out both for its abundance of water and the life it supports. What makes water so special as the medium of life?