How do tattoos work?

Okay, I still don’t get how tattoos can possibly last forever. It is my understanding that every molecule in the human body eventually gets rotated out by cell death or by various chemical processes (supposedly every seven years, although I’ve never heard that from an actual biologist). Yet the dye in a tattoo stays for a lifetime. What’s up with that? Wouldn’t the skin cells containing the dye eventually work their way to the surface and slough off? Or just die and get flushed out? If not, wouldn’t the cells’ mechanism for disposing of waste clear that stuff out? What’s happening at the cellular/molecular level that lets a foreign substance hang out in my skin for decades?

And no, I don’t have one. I’m thinking about it. My wife thinks it would be cool, but my mom thinks it’s stupid. They both may be right.

More than you ever wanted to know about tattoos

And since you’ll eventually want this information anyway…More than you ever wanted to know about getting rid of tattoos