How did Sid help aunt Polly to find out that Tom had been swimming?

Just when Tom thinks he's free and clear of suspicion, Sid points out the color of the thread Aunt Polly used to sew Tom's collar.... busted.

“Some of us pumped on our heads—mine’s damp yet. See?”

Aunt Polly was vexed to think she had overlooked that bit of circumstantial evidence, and missed a trick. Then she had a new inspiration:

“Tom, you didn’t have to undo your shirt collar where I sewed it, to pump on your head, did you? Unbutton your jacket!”

The trouble vanished out of Tom’s face. He opened his jacket. His shirt collar was securely sewed.

“Bother! Well, go ‘long with you. I’d made sure you’d played hookey and been a-swimming. But I forgive ye, Tom. I reckon you’re a kind of a singed cat, as the saying is—better’n you look. This time.”

She was half sorry her sagacity had miscarried, and half glad that Tom had stumbled into obedient conduct for once.

But Sidney said:

“Well, now, if I didn’t think you sewed his collar with white thread, but it’s black.”

“Why, I did sew it with white! Tom!”