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Good, said the judge, rapping his gavel three times. "I always have trouble remembering the long ones. How about 'I am'? That's the shortest sentence I know."
— Norton Juster · The Phantom Tollbooth

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The judge moves from long sentences to the tiny statement 'I am', treating a sentence as something to remember and recite. In a judge's mouth, the word also suggests a punishment. The joke lets the language of grammar intrude on the language of a courtroom.
Context reviewed