Have you ever notice a waxy oil like substance on your face or scalp? I'm not meaning the oil after a big workout or day on the town, but on clean skin after a shower? It's called sebum.
You may not notice any color, or it may appear a bit yellow. It is found on every surface of the body, used to retain moisture in the skin. Our sebum production fluctuates over years starting off incredibly high at birth, which is why we often see babies with cradle cap due to a sebum overproduction. It then declines, and spikes just in time for puberty which undoubtedly contributes to good old high school acne.