Aunt Polly is suspicious of Tom because of his wet hair. During dinner, she asks a series of "leading" questions trying to get him to confess to having skipped school to go swimming:
“Powerful warm, warn't it?”
“Didn't you want to go in a-swimming, Tom?”
A bit of a scare shot through Tom—a touch of uncomfortable suspicion. He searched Aunt Polly's face, but it told him nothing. So he said:
“No'm—well, not very much.”
The old lady reached out her hand and felt Tom's shirt, and said:
“But you ain't too warm now, though.”
Tom keeps avoiding her questions. He claims to have used the water pump on his hair. The tricky part is his shirt collar. Aunt Polly had sewn it. If he got wet with the water pump, it would still be intact. If he had removed his clothes to swim, it wouldn't be.
At first, Aunt Polly is reassured by the look of the sewn color. But it is Sid, Tom's brother, who finally rats out Tom. He points out that the thread is black, but Polly had used white thread. Thus, she knows is has been re-sewn by Tom.