The sun gives life. In addition to warming and lighting the earth, it also helps plants, the bottom rung of the food chain, to grow. Additionally, there is research that suggests exposure to sunlight may help reduce depression, particularly during the winter months. Sunlight is the source of ultraviolet rays, which can be divided into two types, Ultraviolet A (UVA) and Ultraviolet B (UVB). UVB rays help the body produce and process vitamin D, which is necessary for strong bones. Unfortunately, UV rays also are responsible for suntans, sunburns, and premature aging. They also cause skin cancer and eye damage. UVArays penetrate more deeply into the skin and are partially able to penetrate through even heavy clouds, glass, and smog. Ultraviolet exposure over time can cause the destruction of individual cells in the skin and eyes. Sometimes the UV exposure can cause a defect in the DNA of a cell. When that happens, it will keep on dividing and copying the error in the DNA into future versions of itself. The new cells may divide and grow wildly, becoming a skin cancer. Melanin, or the pigment in the skin, absorbs radiation from UV rays. Therefore, it acts as a sunscreen to the cells below it. People with dark skin have a lot of melanin in their skin naturally. Lighter-skinned people must produce extra, which is why they tan and burn when exposed to a lot of sun. However, no matter how much melanin you have in your skin, it is not enough to screen out all the UV rays, and some will damage your skin. You can help limit the damage by using sunscreen.