During adolescence, acne occurs in 80% of girls and 90% of boys.  Pimples form when a duct or pore of the skin becomes blocked.  This is because the amount of sebum (oil) your skin produces increases during puberty.

Blackheads are not dirt.  They are caused by plugs of sebum on the surface of the skin which darken when exposed to air.  Whiteheads are caused when a small amount of pus comes to the surface.

Pimples or acne are not usually caused by eating chocolate or fatty foods, but having a healthy diet will help to maintain healthy, clear skin.