What "trick" did the boys play on each other when they were 15? Explain the circumstances of "Tom's" "joke" and the dramatic irony of his verbal rebukes.

Tom performs the common boyhood trick of feigning panic in the river. When somebody comes to help, they begin to laugh. Tom was supposed to be doing this trick when Chambers rushed out and actually saves Tom from drowning. This humiliates Tom to no end. Of course the reader knows Tom's motivations for verbally attacking Chambers after as do the boys who end up taunting Tom, calling Chambers, "Tom Driscoll's nigger pappy."