Skin

Skin is the soft, stretchy outer covering of your body. It is also the largest organ a human has. If you peeled the skin off an adult and laid it flat, it would cover about 20 square feet, roughly the size of a small bedsheet. It weighs around 8 pounds, more than the brain or the liver. Your skin protects everything inside you and helps you sense the world.

Skin has three main layers. The top layer is called the epidermis. It is the part you can see and touch. The middle layer is the dermis. It holds your sweat glands, blood vessels, and the tiny nerves that let you feel things. The bottom layer is the hypodermis. It is mostly fat, and it cushions your body and keeps you warm.

The outside of your epidermis is actually made of dead cells. New skin cells are born deeper down. They slowly push up to the surface, harden, and flake off. Your whole epidermis replaces itself about once a month. By the end of a year, you have grown several pounds of brand-new skin without thinking about it.

Skin does many jobs at once. It keeps germs and water out. It holds your body's water in. It helps cool you down through sweat, and it warms you up by trapping heat in the fat layer. It even makes vitamin D when sunlight hits it. Tiny nerve endings in your skin send signals to your brain about heat, cold, pressure, and pain. That is how you feel a soft sweater, a hot stove, or a tickle on your arm.

Skin color comes from a substance called melanin. Cells in the epidermis make melanin, and the more they make, the darker the skin. Melanin also helps protect skin from the sun's strongest rays. Every person on Earth has these cells. The amount of melanin they make is what creates the wide range of human skin colors.

Skin can heal itself. When you get a small cut, blood quickly forms a scab to seal the wound. Underneath, new cells grow to close the gap. Sometimes the new skin looks slightly different from the old skin, and that mark is called a scar. Deeper cuts and burns leave bigger scars because the skin cannot rebuild every layer perfectly. Even so, the way skin patches itself up after almost every scrape is one of the most useful tricks your body knows.