"Put on your sunscreen!" This familiar phrase can be heard at most beaches on a sunny day. It's an important directive, though, because sunlight, for all of its beneficial effects, can readily damage the skin. The most dangerous wavelengths of sunlight are the ones we can't see: the ultraviolet (UV) region of the spectrum. Not only can excess exposure to UV light damage skin cells, it can also cause a deadly form of skin cancer that kills nearly 10,000 Americans each year.

Why is UV light so dangerous? How can these particular wavelengths of light damage our cells to the point of causing cell death and cancer? As you read this chapoter, look for clues to help you solve the question of why UV light is so damaging to skin cells. Then, solve the mystery.