What does that mean? "It comes down to the very fundamental principles of physics and chemistry," says Dr. Schultz. "After you apply your first product, the second product has to have the ability to go through it in order to penetrate the skin." In other words, the first things you put on your skin should be the lightest texture. Then, work your way up to the heaviest, hard-to-penetrate textures, which should be applied last. So, anything that is water- or alcohol-based should come first, followed by your gels, then your light lotions, then heavier creams, thick serums, and, finally, anything ointment-based (i.e. water-free). One way to tell how heavy the product is? Dr. Schultz says to read the ingredient list. Find out where the oils, lanolins, and petrolatums fall — the higher up they are, he says, the harder the product is to penetrate, and the later in your routine it should go.