Your skin’s an amazing, complex organ that extends much deeper than we can see. It’s your largest and heaviest organ.Skin makes up about 15 percent of your body weight, covering 12 to 20 square feet. What’s it made of? Seventy percent water, 25 percent protein and less than five percent fat.Its obvious role is to protect your insides—your organs, blood and bones from the outside elements. And it holds your body together.Skin also helps with healing. Touching in a loving way reduces the stress hormone cortisol, and boosts the feel-good hormone, oxytocin. Massaging and caressing also stimulates the vagus nerve, which runs to the brain and improves the health of the body.Aside from acting as a barrier to chemicals and germs, skin has a major sensory function. Follicles grow hairs that can sense stimuli before your skin is even touched. For example, eyelashes cause the eyelid to involuntarily close to protect the eye.QUIZ: How Healthy Is Your Skin?Your skin also lubricates itself with oils. The sebaceous glands produce sebum and also absorb certain medications and hormones/health/hormone-levels. Skin can also absorb toxins, which you don’t necessarily want. Ultraviolet light can turn your skin against itself, creating those ever-talked-about, damaging, free radicals. UV rays can also change your DNA, and not for the better.Like many body structures, your skin contains several components.Epidermis: The primary barrier against the outside is less than a millimeter thick. Only the right-sized molecules can get through. It also renews itself every six to eight weeks. How? Dead cells continually slough off, and new ones emerge from below. (Yep, that’s how we get so much dust in the air.) Your epidermis largely affects how fresh your skin looks, and how well it absorbs and retains moisture.Dermis: The thickest of your layers, the dermis holds you together. It’s made up of fibroblast cells, which make collagen and elastin. These are proteins that give skin strength and elasticity. Dotting the dermis are hair follicles, sweat glands and sebaceous glands (which produce the oily sebum that lubricate your skin and hair). Sebum is a mixed blessing—it keeps bacteria under control, but also attracts insects. Lastly, the dermis contains tiny blood vessels (to nourish) and lymph nodes (to protect from toxins).Subcutaneous tissue: The innermost layer is made of fat and acts as a shock absorber and heat insulator. Many mammals don’t have this layer because their fur does the same job.