If you get right down to it, starch is a tiny molecule

used by plants to store energy, but, more important,
it’s a vital tool in the culinary arsenal, used to
thicken gravies, add body to stews, and prevent
sauces from breaking and turning greasy. There are
two basic varieties. Amylose resembles a long
straight chain, made up of thousands of glucose
molecules. Amylopectin resembles a small clump of
weeds—bushy, with many entangled branches. When
dispersed into a liquid, the long amylose molecules
have a tendency to get tangled up with one another,
sticking together and forming a loose matrix that
adds viscosity. Amylopectin will do the same thing,
but since it’s so compact, it does so less efficiently.
The amylose and amylopectin contents of various
starches determine their thickening power.