Siblings: children of the same parents, each of whom is perfectly normal until they get together.

The joke hinges on the contrast between how people appear individually (“perfectly normal”) and how group dynamics—especially among siblings—can trigger rivalry, regression, teasing, and old roles. Levenson implies that “normality” is partly a social performance: alone, each child can seem reasonable, but together they revert to a private family language of competition and provocation that outsiders rarely see. The mock-definition format also satirizes the idea that family relationships are simple or inherently harmonious; instead, intimacy breeds a special kind of chaos. Beneath the humor is a sharp insight about identity: siblings often know exactly how to push one another’s buttons because they share history, status contests, and parental attention.