The skin's surface, with its many micropeaks and valleys, scatters a small number of the sun's ultraviolet rays. Most of the rays penetrate the epidermis. Melanin absorbs many of these photons, but some of the remainder injure DNA in the living cells. This damage signals capillaries in the dermis to dilate, increasing blood content and producing redness, or sunburn. Sunscreen acts as
a filter that absorbs additional UV rays, dissipating their energy. Tanning oil smooths the surface, so fewer rays scatter and more penetrate, speeding sunburn or tanning.