Every pore of your skin contains a small gland that produces a fatty liquid called sebum. It is essential to the balance of your skin as it lubricates and protects it against harsh external factors. But in adolescence, sebum production gets carried away under the effect of hormones which make it more irritating. As a result: too much irritant sebum spreads over your skin, which becomes oily and shiny and starts becoming inflamed. This is the first stage towards the appearance of blackheads and pimples.