How did Aunt Polly try to discover if Tom had gone swimming?

Tom was supposed to go to school, but as it's such a lovely hot summer's day, he decides to play hooky and go swimming instead. Aunt Polly is suspicious and tries to get Tom to own up at dinner later that evening. But Tom adamantly insists that he didn't play hooky. If he'd gone swimming, he says, he would have had to remove his shirt collar, undoing Aunt Polly's sewing. But look—the collar's seam is unbroken, and so Tom couldn't have gone swimming.
Aunt Polly apologies to Tom for her suspicions. But trust Sid the snitch to mess things up for his cousin. He notices that Tom's shirt collar is being held together by white thread, even though Aunt Polly used black thread when she sewed it that morning. Tom's plan has been rumbled; he used white thread to sew his collar back on after finishing his swim, thinking he'd be able to fool Aunt Polly. But thanks to Sid's eagle eye, Tom is caught in a lie.