Gelatin is the easiest, most forgiving protein any cook deals with. Heat it up with water and its molecules let go of each other and become dispersed among the water molecules; cool it and they rebond to each other; heat it again and they disperse again. Nearly all other proteins in animals and plants behave in exactly the opposite way: heat causes them to unfold from their normally compact shape, become entangled, and form strong bonds with each other, so that they coagulate permanently and irreversibly into a firm solid. Thus liquid eggs solidify, pliable muscle tissue becomes stiff meat, and milk curdles. Of course a solid piece of coagulated protein can’t be a sauce. But it’s possible to control protein coagulation so that it can give body to sauces.