Input (grade 13): According to the plot, during a period where Aang was young, he unknowingly revealed he was the Avatar when he chose four toys out of thousands, each of which were the childhood toys of the previous Avatars.
Output (grade 12): When Aang was young, he unknowingly revealed that he was the Avatar when he chose four toys out of thousands, each of which were the childhood toys of the previous Avatars.