First things first is that wrinkles and a creepy appearance happen for the same reason, no matter where it is on the body: Loss of collagen and elastin, thinning dermis, and cell damage from environmental stressors like the sun. The knees are no different.

However, what makes the skin on the knees unique is their function. The skin over the knees experiences tons of movement and stretch. It’s not necessarily the movement that’s the problem—creases and lines are part of a well-functioning face—but as you lose vital skin components like collagen, your skin isn’t as able to bounce back from these movements. 

It’s actually the same reason places like your mouth, eyes, and forehead develop fine lines before more stationary parts of the face. 

“Over time, these lines will deepen and etch into the skin, similar to the way folding paper will leave a crease,” says board-certified dermatologist Cynthia Bailey, M.D., founder of Dr. Bailey Skin Care.