After the history test, Mr. Johnson called Johnny over and placed his paper beside Mary’s. Both students had identified George Washington as the first president. Johnny protested: everyone knew that. The teacher then pointed to their identical answer about who freed the slaves: Abraham Lincoln. Again, Johnny had a perfectly reasonable defense. He had studied his history book the night before. Two correct answers, he insisted, hardly proved he had copied anything from his neighbor.

Mr. Johnson moved to the question about who was president during the Louisiana Purchase. This time, Mary had offered no famous name, just an admission: “I don’t know.” Johnny’s answer beneath the same question was only two words: “Me neither.” Suddenly, his argument about common knowledge had nowhere left to go. He hadn’t merely failed to name a president; he had answered Mary instead of the question. The evidence he demanded was right there in his own handwriting.