Your skin acts as an indicator of what is going on in your body. For example, your skin turns yellow when you suffer from jaundice, or it may turn red when you suffer an allergy to some drug or food. If you notice any odd changes in your skin, it could be a sign for concern. Redness accompanied by swelling of the upper arm, the thigh bone, or the shin bone (particularly during adolescent years) could be a sign of an osteosarcoma--a form of bone cancer. [caption id="" align="aligncenter" width="493"] Source: webmd.com[/caption] Then there is the classic cancerous mole, or dark spots on the skin. Sun spots are fairly common among people who spend a lot of time in the sun, but if you notice that the spot is changing color--growing darker--it could be a sign of skin cancer. If you have a mole or a sun spot that grows or changes shape, it could also be a sign of melanoma. Time for a biopsy!