Water is the most miraculous of fluids. As well as being ubiquitous on Earth and essential for life as we know it, it has remarkable properties which at first sight don’t seem to be consistent with its almost laughably simple chemical composition. Each molecule of water consists of a single oxygen atom (O) and two hydrogen atoms (H); its chemical formula is therefore, as just about everyone already knows, H2O. Here is one odd but hugely important anomalous property. A water molecule is only slightly heavier than a methane molecule (CH4; C denotes a carbon atom) and an ammonia molecule (NH3, N denotes a nitrogen atom). However, whereas methane and ammonia are gases, water is a liquid at room temperature. Water is also nearly unique in so far as its solid form, ice, is less dense than its liquid form, so ice floats on water. Icebergs float in water; methanebergs and ammoniabergs would both sink in their respective liquids in an extraterrestrial alien world, rendering their Titanics but not their Nautiluses safer than ours. Another very important property is that water is an excellent solvent, being able to dissolve gases and many solids. One consequence of this ability is that water is a common medium for chemical reactions. Once substances are dissolved in it, their molecules can move reasonably freely, meet other dissolved substances, and react with them. As a result, water will figure large in this book and this preliminary comment is important for understanding what is to come. You need to get to know the H2O molecule intimately, for from it spring all the properties that make water so miraculous and, more prosaically, so useful. The molecule also figures frequently in the illustrations, usually looking like 1, where the red sphere denotes an O atom and the pale grey spheres represent H atoms. Actual molecules are not coloured and are not made up of discrete spheres; maybe 2 is a better depiction, but it is less informative. I shall use the latter representation only when I want to draw your attention to the way that electrons spread over the atoms and bind them together.