The only type of can you'll be building in prison is one made of pottery. Do you honestly think they would let you have anything made of metal or plastic? No, too dangerous. You have to make all of your cans and plates and underpants the old-fashioned pottery way.
You get so good at pottery that the guards start to notice and offer to sell what you make in とうきょう. とうきょう is a big pottery town, after all. You'll make a fortune! Well... the guards will at least.