9. Getting A Tattoo
https://www.youtube.com/watch?v=kxLoycj4pJY Tattoos are pretty cool, and not just because they make you look tough. A tattoo machine works by injecting ink through the epidermis (the outer layer of skin) and into the dermis. The moment the ink enters your body, your immune system will kick in to try and get rid of it, just as it would with a pathogen. The reason why tattoos are permanent is that the particles of ink are larger than the white blood cells that are trying to get rid of them, so they stick around. The smaller particles are removed by the white blood cells, which is why tattoos fade. Skip to 3:10 for the actual slow mo footage. Also, take a look at how tattoos are removed again, jus in case you end up regretting you back-piece dedicated to your high school girlfriend. https://www.youtube.com/watch?v=D0B7F5UbTOQ