A larger amount of male hormones in the body cause changes to perspiration (sweat). This gives it a more "adult" body odor or smell. The hormones also cause sebaceous glands in the skin to make more sebum, a type of oil that keeps the skin from getting too dry. Sebum and dead skin cells can mix with a type of germ called bacteria and block pores (tiny openings) in the skin. This causes a skin problem called acne. Most people get acne during puberty. When a person has acne, he gets swellings on his skin called blackheads, whiteheads and pimples. Blackheads and whiteheads are painless, but pimples can be red and painful. They usually form on the face but can also appear on the neck, shoulders, upper back and chest. Oil glands on scalp – the part of the head on which hair grows – also make a lot of sebum. This can cause dandruff, in which small white bits of skin fall from the scalp.