It's a beautiful hot, sunny day in St. Petersburg; certainly not the kind of day you'd want to spend in school. Mind you, Tom Sawyer never wants to spend any day in school, whatever the weather. He's always looking for an excuse to play hooky and the hot weather gives him a great opportunity to skip school and go for a nice refreshing swim instead. There's just one problem: If Tom's to go swimming, he needs to take off his shirt, and if he's to take off his shirt, he needs to sew the collar back on before he heads off home.

Fortunately, Tom's already got this covered—or so he thinks. After finishing his swim, he carefully sews the collar back into place. This way, Aunt Polly will never suspect that he played hooky. Nevertheless, she's still understandably suspicious at dinner that evening; she knows what her mischievous scamp of a nephew's really like. Tom vehemently protests his innocence, pointing to the sewn shirt collar as evidence that he's been as good as gold. Aunt Polly apologizes to Tom for jumping to conclusions. Unfortunately for Tom, though, sneaky Sid the snitch is on hand to expose his subterfuge. He calls Aunt Polly's attention to the black thread that's holding Tom's collar in place. Whereas, when Aunt Polly had sewn it earlier, she'd used white thread. Aunt Polly now knows that Tom played hooky to go swimming, and the young scamp's in serious trouble.

The answer to your question comes in chapter one. Tom, who has already proven himself to be something of a rapscallion, skips school to go swimming, and his aunt Polly is immediately suspicious. She checks his shirt collar, which she had sewn that morning. Seeing that the sewing is still intact, she is momentarily pleased to think that perhaps he has, for once, been obedient.

However, to Tom's detriment, his half-brother Sid points out to Aunt Polly that while she had sewn the collar with white thread, it is now sewn with black thread.

For the second time in this chapter, Tom runs out of the door in order to avoid punishment, but not before threatening to beat Sid up upon his return.

Before Tom left for school in the morning, Aunt Polly had sewn his shirt collar with white thread. At dinner, Sidney notices that Tom has sewn his shirt collar with black thread instead of white.