Liquid water (H2O) is often perceived to be pretty ordinary as it is transparent, odorless, tasteless and ubiquitous. It is the simplest compound of the two most common reactive elements in the Universe. However, it is the most remarkable substance. Although we drink it, wash, fish and swim in it, and cook with it (although probably not all at the same time), we nearly always overlook the special relationship it has with our lives. Droughts cause famines and floods cause death and disease. We are about two-thirds water and, without it, we die within a few days. Life cannot evolve or continue without liquid water, which is why there is so much fuss about water being found on Mars and other planets and moons. It is unsurprising that it plays a central role in many of the World's religions. Because of its clear importance, water is the most studied material on Earth but it comes as a surprise to find that its behavior and function are so poorly understood (or even ignored), not only by people in general, but also by scientists working with it everyday. Water seems, at first sight, to be a very simple molecule, consisting of just two hydrogen atoms attached to an oxygen atom. Indeed, there are very few molecules that are smaller or lighter. The size of the water molecule, however, belies the complexity of its actions and its unique capabilities and anomalous properties seem to fit ideally into the requirements for life as can no other molecule. It is clear that life on Earth depends on the unusual structure and anomalous nature of liquid water. Organisms consist mostly of liquid water. This water performs many functions and it can never be considered simply as an inert diluent; it transports, lubricates, reacts, stabilizes, signals, structures and partitions. The living world should be thought of as an equal partnership between the biological molecules and water. many of the properties of water are puzzling. Enlightenment comes from an understanding that water molecules form an infinite hydrogen-bonded network with localized and structured clustering. The middling strength of the connecting hydrogen bonds seems ideally suited to life processes, being easily formed but not too difficult to break.