Most of the external surfaces of our bodies including the skin and hair contain a pigment called melanin. As a result, the skin usually gets tanned. Because we each have different levels of melanin production, the sun does not have the same affect on everyone. Fair skinned people with red or blond hair have less melanin, so they tend to burn rather than tan. For one thing, ultraviolet rays tend to destroy the elastic fibers in the skin which help to keep it taught.
Wrinkles become more common and the skin ages prematurely, developing a hard leathery feel.
Because ultraviolet rays also destroy the DNA found in skin cells, the cumulative affect of long term sun exposure is the primary cause of some potentially deadly skin cancers. During the warmer months, students can set up a controlled experiment to test the effectiveness of different sun screens and report on their results. The cavies are a family of South American rodent which includes the common guinea pig as well as the giant Patagonian cavy. All cavies are social animals, living in burrows and eating primarily grass and other leafy vegetation. Most cavies including the guinea pig are nocturnal however, the Patagonian cavy spends most of its time feeding and basking in the sunlight.