Today’s burning question is “what causes crazing in pottery”.
The answer: clay and glaze, like pretty much the rest of the universe, expand and contract depending on temperature, atmospheric pressure, etc. Not enough that you’d notice it without a microscope, but it happens. The thing is, the clay and the glaze are doing it at slightly different rates, and that creates stress. Over time, cracks form in the glaze. A potter can try to minimize crazing, by formulating a glaze that moves at close to the same rate as the clay, but it’s impossible to match the expansion rate exactly.
 Unless someone has actually managed to invent a magical mug that grows bigger every time you pour tea into it. I’d totally buy one of those.