Skin Does More Than Cover

It is easy to think of skin as wrapping. It is closer to a multitasking organ:

• Barrier: dead surface cells seal water in and keep germs and UV out.
• Chemist: it makes its own signaling molecules and helps produce vitamin D.
• Sensor: touch receptors packed unevenly, so some spots feel fine detail others miss.
• Shield that adapts: melanin tunes itself to the sunlight your ancestors faced.

One sheet, doing four very different jobs at once.