What other “necessary” task is Jim required to do with nails in order to “be correct” in escaping? CH 34-38

Tom designs a coat-of-arms for Jim to inscribe on the walls so as to permanently leave his mark on the prison cell. Next, Tom works out three mournful inscriptions and tells Jim he must carve them into a rock. Huck and Tom go to fetch an old grindstone for Jim to use as his rock, but it is too heavy for them to carry, so they are forced to allow Jim to leave his "prison" and come help them. Jim rolls the rock into the hut and sets to work on the inscriptions.Tom has no sense for practicality. He wants Jim's escape to resemble an escape adventure story instead of just setting Jim free.