Burns are classified according to the severity of the injury to the skin and underlying tissues. The three burn categories are first-degree, second-degree, and third-degree burns, with first-degree burns being the most minor. If the first and second layers of skin have been burned, you have a second-degree burn. Second-degree burns can sometimes be considered minor. For both first- and second-degree burns, apply cool running water to the area for at least 10 minutes. This will help reduce swelling. The most serious burns are third-degree burns. Not only are all the layers of skin burnt, but muscle, fat, bone, and other tissues are temporarily or permanently affected. A third-degree burn requires medical attention. Another serious burn that may need treatment is an electrical burn, like from a power cable. These types of burns may cause internal damage, even if the burn does not leave a mark on the skin.