Water is the most common liquid on our planet, vital to all life forms. It is the dispersion medium for all biochemical reactions of the living process and takes part in many of these reactions. In spite of the chemical simplicity of the water molecule, its physical properties are quite remarkable -- one might say weird! -- and have been a major research topic for many years. Many experiments give great insight into the structure and dynamics of water, but these often require the resources of a major research laboratory. In addition, some must take place under extreme conditions, such as those leading to superheated or supercooled water.
Water has been studied since antiquity. What is new is that computers now allow us to start with elementary interactions between molecules and from them predict the large--scale properties of water, such as pressure, temperature, volume, solubility of salts, and so forth. In other words, computers can be used to calculate the physical quantities of water related to every day life.
The computer simulation Wasser (the German word for ``water'') has made a major contribution to research on the behaviour of water. The addition of a graphical interface that pictures the atoms helped to make the Wasser program usable for students and increased its usefulness for research workers. The computer simulation not only correctly predicts macroscopic properties in agreement with experiment, but also allows us to investigate water under severe experimental conditions that are accessible only with great difficulty or not at all.