The indentations at the underside of cups
You might have noticed that several, if not all, your cups have these little grooves on the bottom, and the reason is to make them easier to wash. If there were no indentations, then after placing them upside down to dry, the water would pool.
However, that is not their only purpose. They allow temperature air to flow below your cup, so when you place a hot drink on it, it does not break. Now, we wonder who figured out the problems and how they went about to fix them.