I saw years ago about a uv ink. So the tattoo is only visible under black light. I always wanted to get a tattoo with that ink of a ninja. So when people ask if I have any tatts I can say "yes, a ninja" and roll my sleave up. They say "there's nothing there"
I reply "he's a very good ninja"
Of course I have neither the inclination or money to actually go through with this.