"I lost my helmet somewhere in the house trailer just before the flashover hit," says Stevens. "It was about 2,000 degrees Fahrenheit (1,000 degrees Celsius) in there when I jumped out the window."

Five years ago Stevens was a volunteer firefighter. Now, preparing for his sixth major reconstructive surgery, he laughs. "I'm learning more about skin than I ever wanted to know."

If you took off your skin and laid it flat, it would cover an area of about 21 square feet (1.9 square meters), making it by far the body's largest organ. Draped in place over our bodies, skin forms the barrier between what's inside us and what's outside. It protects us from a multitude of external forces. It serves as an avenue to our most intimate physical and psychological selves.

This impervious yet permeable barrier, less than a millimeter thick (0.04 inch) in places, is composed of three layers. The outermost layer is the bloodless epidermis. The dermis includes collagen, elastin, and nerve endings. The innermost layer, subcutaneous fat, contains tissue that acts as an energy source, cushion, and insulator for the body.

From these familiar characteristics of skin emerge the profound mysteries of touch, arguably our most essential source of sensory stimulation. We can live without seeing or hearing—in fact, without any of our other senses. But babies born without effective nerve connections between skin and brain can fail to thrive and may even die.

When a blast of cold air hit our ancestors tens of thousands of years ago, their body hairs stood on end, fluffing up the hair and trapping more air between the hairs to warm the body, much like a down blanket. Today we call this reaction goosebumps, and it still occurs when tiny muscles, called piloerectors, located in each hair follicle, contract and raise body hairs. Even though we've become progressively less hairy over the millennia, our bodies continue to respond to cold in the same way.

So why do we then get goosebumps when we're scared or frightened? When you become emotional, your nervous system automatically responds by sending hormones that trigger a "fight or flight" response that also causes your hairs to raise. This effect may have helped us ward off enemies by looking bigger and tougher. Other animals, like cats and dogs, continue such displays today when fighting for dominance.

— Christy Ullrich