Water is one of the most important ingredients and tools in the kitchen; its influence is everywhere. Paying attention to the properties and effects of water and knowing how to use them, from its boiling point (which, at sea level, is like a built in thermostat set at 212 degrees F.), to its density, to its capacity to evaporate, to its cooling the contents of a pot, is a fundamental part of cooking.
[Update: See Pardus's comment, 11/27, on results of his stock test if you've been following that part of the thread.]
This is an element, as we’ve seen from the previous post, about which whole essays could be written. While previous comments focused on the quality of tap water versus distilled or filtered—one part of water’s role as an ingredient—what interests me most, and why I included it in Elements of Cooking, is its role as a tool.
Water is a cooking tool when you use it to regulate temperature. One of the most common water techniques is the bain marie, or water bath, in which we cook custards or other delicate concoctions that would be hurt by too much heat. When we put a ramekin in a water bath and bake it, that ramekin will stay below 212 degrees thanks to the water.
An ice bath will cool down a pot rapidly and it will stop a green bean from cooking.
Keeping the effects of water in mind will help you sauté better. The browning of food adds flavor to the food. But food won’t brown at low temperatures/when water is present. So if you want a piece of meat to brown, make sure its surface is dry. If you want mushrooms to brown, they have to go into a pan that will brown them before they begin releasing their water. Water is fundamental to life and fundamental to cooking.