The way some people describe the human body, it might be thought that inside it there are some bones, a few organs, one or two other bits and pieces, and all are floating in a vast sea of internal fluids. Yet that is not as ludicrously exaggerated as it may sound. The body is, after all, more than two-thirds liquid in overall volume. Of course it is not just one sort of liquid---blood is obviously the major form, but there are others, some as vital. Almost all have one constituent in common; water. Even blood plasma is ninety per cent water. So we all have to drink water in quantity.