What's the most amazing part of the human body? (inside or outside)

You decide what amazing is.
So what part of the human body do you find the most fascinating? Hair, legs, intestines, the heart, the brain, the eyes, teeth, lungs, nipples? What, and why?

I think the liver is pretty cool, because it actually regenerates itself when damaged. That’s actually kind of creepy, but real fascinating at the same time. I also love hair, especially long hair. You?