What concession to reality does Tom make in his plan to dig out Jim?

The next night, Tom and Huck sneak out and start digging with their case knives. They tire soon and their hands quickly develop blisters, but it seems they haven't accomplished anything. Tom finally sighs and agrees to use a pick and shovel, but only as long as they pretend to be using case knives. Huck agrees and tells Tom his head is getting "leveler" all the time.