Good question, reader Luther! It’s commonly said that flour granules “pop” in a hot, wet environment, releasing their starches and creating thickening. But that’s not quite right. Think of a flour granule as a tightly-bound bundle of reeds and branches (amylose and amylopectin [starch] molecules), a broken-off chunk of wheat berry endosperm. Immerse that granule in hot water and the water molecules start to intrude into the mass, prying off the individual starches. Those molecules then disperse into the liquid.
Gelatin proteins behave in a similar way. However the big difference between starches and proteins is that starches don’t bond with one another like proteins do. Thus the “net” that they make is really more like a tangle, a logjam as it were, that keeps the water within from flowing. And when water molecules can no longer flow, you get thickening.
Continuing with the logjam analogy, it’s the combination of lots of little twigs and branches (the individual molecules) and big logs (the mostly-undissolved granules) that causes thickening. If the logs all broke down into little twigs, the current of the river would wash them all away. This is essentially what happens when you heat a flour-thickened sauce for too long. It reaches optimal thickness just as it’s coming to a boil, where the proportion of logs and twigs is just right for stopping liquid flow. Beyond that point the sauce begins to thin out again as the logs begin to break into pieces and are carried away in the stream.