Looking forward to getting some sun during the long weekend? You may already know that sun rays bring UV radiation, which damages skin cells and will eventually lead to skin cancer for two million Americans every year. But if you still aren't convinced to slather on sunscreen or wear a hat, consider this: radiation continues to damage your skin on a molecular level for hours after initial sun exposure. Through a series of experiments with both mice and human skin cells, dermatology professor Douglas Brash of the Yale School of Medicine found that sun damage chemically changes DNA in melanocytes -- the type of skin cells that produce melanin, which gives your skin pigment and protects it from sun damage by absorbing sun rays -- for two to four hours after UV exposure is over.