Beneath the epidermis is a layer of skin called the dermis. This layer has sweat glands and sebum glands. Sebum is an oil that makes you waterproof. The sweat and sebum from the glands travel to the surface of the skin through narrow channels called pores. So, every second of the day, your body is outwardly radiating thousands of different cells through your pores, and your pores deserve a lot of credit for providing safe passage for these cellular nomads and enabling them to exit your body. All of these dead skin cells, oily sebum and sweat form a protective mix (along with other lipids, such as linoleic acid) on the skin's surface. This shield stops your skin from drying out and prevents harmful bacteria from infiltrating your skin. Males actually produce more sebum than females, but both genders crank up the sebum levels during puberty. This spike in sebum production is why puberty and acne form an evil alliance.
The innermost layer of skin is called the subcutaneous layer. This is where your hair begins its growth, in the bottom of a porelike tube called a follicle. As hair grows out of this follicle, it makes its way through the other layers of skin and collects a coating of sebum in the process. This keeps it shiny and water-resistant. This is also why your hair gets oily and sticks out from your head in crazy directions if you go a few days without washing it.
Just like a car crash in a tunnel will cause a major traffic blockage behind it, so too does a jam-up in a pore or follicle. We'll talk about these cellular crashes and oily wipeouts in the next section.