You rinse, pat dry, pick up the serum, and it sits on the surface. Two or three passes later it finally disappears. By lunchtime the same patch on your cheek feels tight again. The usual next move is a heavier serum, or a bigger pump of the one already open. That tends to make the sliding worse. What changed is the surface it lands on. A minute after you wash, your skin is drier than it was before you washed, and everything that goes on after that has to work against it. What happens in the...