You put a tattoo on yourself with the knowledge that this body is yours to have and enjoy while you're here. You have fun with it, and nobody else can control (supposedly) what you do with it. That's why tattooing is such a big thing in prison: it's an expression of freedom—one of the only expressions of freedom there. They can lock you down, control everything, but 'I've got my mind, and I can tattoo my body—alter it my way as an act of personal will.'