How did Tom convince Sid that he was dying?

Tom used everything he had to produce the groans of a dying boy, handed out forgiveness, and even suggested where his belongings should go.

“Oh, Tom, you ain’t dying, are you? Don’t, Tom—oh, don’t. Maybe—”
“I forgive everybody, Sid. [Groan.] Tell ’em so, Sid. And Sid, you give my window-sash and my cat with one eye to that new girl that’s come to town, and tell her—”
But Sid had snatched his clothes and gone. Tom was suffering in reality, now, so handsomely was his imagination working, and so his groans had gathered quite a genuine tone.
Sid flew downstairs and said:
“Oh, Aunt Polly, come! Tom’s dying!”