Because active ingredients are more expensive than thickeners, serums are also the costliest product in many skin care lines. But when applied properly, a 1-ounce container of serum should last months. Many brands come in pump bottles or have medicine-dropper applicators to dispense just the tiny amount you need.

Those few concentrated drops are super-efficient. Serums are made of very small molecules, so the skin absorbs them quickly and deeply. "The thicker, heavier ingredients in creams form a barrier on your skin," Wilson says. "That's great for locking moisture in. But it can also lock active ingredients out. Without these hindrances, the active ingredients in a serum penetrate your skin faster and more effectively." -Ni'Kita Wilson, a cosmetic chemist and vice president of research and innovation at Englewood Lab.