When the narcissistic gang from "It’s Always Sunny in Philadelphia" visited the "Abbott Elementary" school in a crossover episode earlier this year, audiences got to see the two Philly TV universes collide in hilarious ways — all while keeping the episode PG for "Abbott’s" more family-oriented viewers.

It's Always Sunny in Philadelphia returned with a crossover with Abbott Elementary, and its most shocking moment is explained by one star.

Season 17, Episode 1 of FX's "It's Always Sunny in Philadelphia," marks part two of its crossover event with ABC's "Abbott Elementary."