Most of the time, when we’re talking about thickening in the kitchen, we mean the thickening of watery substances: broth, juice, milk, thin syrups, that sort of thing. Thickening is necessary if we want those substances to have much texture other than…watery. The obvious question here is: why can’t water itself have a more interesting texture? Does it have to be so, well, watery? And what makes it that way? The answer is that water flows because its molecules are so incredibly small. They’re made up of just three atoms: two hydrogens and one oxygen if I recall correctly. Dihydrogen monoxide.
Being as small as they are, water atoms tumble over one another rather easily. Which is to say, they flow. Thickening is the process by which we cooks slow down that flow, maybe even come close to stopping it. This we do by adding large molecules or other bits of stuff to the medium. Water molecules bump into those big things, and that restricts their motion.
All sorts of things can be used to thicken: proteins, starches, fats, sugar and larger particulates of various kinds. All of them thicken a little differently and in the process create different textural effects. I’ll go over as many as I reasonably can in this series of posts!