Your body is constantly throwing off dead skin cells—a million every day. So how come tattoos don't fall off with them? It turns out, your body's immune system tries to get rid of the ink under your skin, but in the process, it makes those marks permanent, as this video from TED-Ed explains.
Funny how that works—your body tries to get rid of something, but only succeeds in making it hang around for the rest of your life. Joke's on you, dermis! [YouTube via Digg]