What is topological sort?

Example: In 1962, Robert Gagné published a study of nine-year-olds learning to add integers in columns. He didn't just teach addition. He mapped the prerequisite hierarchy — showing that to add columns of integers, a child must first be able to add two single-digit numbers, which requires understanding the concept of addition as combining sets, which requires being able to identify the numerals, which requires understanding that symbols represent quantities. When Gagné tested children who had skipped intermediate prerequisites, their failure rate was predictable and nearly absolute: 95% of students who lacked an immediate prerequisite failed to learn the target skill. But when all prerequisites were in place, success rates climbed above 90%. The order wasn't a pedagogical preference. It was a structural requirement of the knowledge itself.

This concept is part of Relationship Mapping in the How to Think curriculum.