Burns that affect only the superficial skin layers are known as superficial or first-degree burns.[2][11] They appear red without blisters and pain typically lasts around three days.[2][11] When the injury extends into some of the underlying skin layer, it is a partial-thickness or second-degree burn.[2] Blisters are frequently present and they are often very painful.[2] Healing can require up to eight weeks and scarring may occur.[2] In a full-thickness or third-degree burn, the injury extends to all layers of the skin.[2] Often there is no pain and the burnt area is stiff.[2] Healing typically does not occur on its own.[2] A fourth-degree burn additionally involves injury to deeper tissues, such as muscle, tendons, or bone.[2] The burn is often black and frequently leads to loss of the burned part.[2][12]