Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent six days a week. I am a clumsy guy. If there are sharp corners nearby, I’ll bash into them. If there’s a surface underfoot with even a light sheen of polish, I’ll take a tumble. You don’t need to take my word for it. A quick look at my knees, which have become knitted with a patchwork of small scars, tells the story. I can trace some of these marks back years, and have accepted that they will be on my body for life. But what gives? Why don’t our bodies remove old scars? The answer goes to the heart of how our bodies have adapted to protect us. Why do some injuries not cause scarring? “The skin is our protection against the external environment ,” says Dr. Corey Maas , an associate clinical professor at the University of California, San Francisco and founder of the Maas Clinic. “It’s a remarkable organ. It’s very important that its integrity be maintained.” The skin consists of three layers.…