. >> at kern valley state prison in california, we met inmate aaron yost who participates in anotherery common practice behind bars. tattooing. >> mine's art work. you know what i mean? i did this myself. >> where? >> in a cell like this. >> how? >> how? see this cassette player? i ripped the motor out of it, and i attach a tube to it with the needle, spin the needle back and forth. and i make the ink. burn plastic or baby oil, and the soot will go up. i collect the soot and mix it with shampoo to make a thick black ink. >> tattooing in prison is illegal and poses a variety of health risks, but we've met countless inmates who have gotten their ink both on the outside and inside. many tattoos represent gang affiliation and are documented by prison officials. most of the rest have a meaning all their own. >> what my trip is is physics. my tattoos represent it. on the back of my legs it says quantum physics. i've got an atom on my hand. i study what the ultimate nature of reality is. it sooths me. it soothes me to try and figure out what i am, what all this is, what it's all about. you