Why does Tom pretend to have a toothache? How does Tom ease his conscience?

Tom pretends to have a toothache so he can tie up his jaws every night and feel safe that Sid won't hear him talk in his sleep.

Tom got out of the presence as quick as he plausibly could, and after that he complained of toothache for a week, and tied up his jaws every night. He never knew that Sid lay nightly watching, and frequently slipped the bandage free and then leaned on his elbow listening a good while at a time, and afterward slipped the bandage back to its place again. Tom's distress of mind wore off gradually and the toothache grew irksome and was discarded. If Sid really managed to make anything out of Tom's disjointed mutterings, he kept it to himself.