When greasy secretions from the skin’s sebaceous glands plug the pores, zits pop up, says Charles E. Crutchfield III, M.D. Clinical Associate Professor at the University of Minnesota Medical School. Common acne in teenagers is caused by high levels of androgens, which are male sex hormones (such as testosterone). Testosterone signals the body to make more sebum, a waxy substance that helps retain skin’s moisture. When excess sebum irritates the skin, it causes redness and swelling–a pimple. Plugges oil glands then form blackheads and whiteheads. When clogged, follicle walls break, and sebum leaks into nearby tissue. And that, bub, is why kids call you pizza face.