This week's "Abbott Elementary" episode is just half of the story. "Volunteers" featured the long-awaited crossover with FX's long-running "It's Always Sunny in Philadelphia," as told from the vantage point of the hit ABC sitcom.

A crossover television event is always a thrilling prospect—not one, but two of your favorite shows coming together as one? There have been plenty of surprising combinations throughout the years: The Jetsons met The Flinstones,

Don’t miss the unexpected twist when Ava announces that a group of volunteers from the school district will be coming to lend a hand at Abbott Elementary.