The Grim Adventures of Billy and Mandy: Which Came First?/Substitute Teacher

"Which Came First?" A field trip to the museum goes unexpectedly when the school bus gets a flat tire in the desert. While exploring, Billy is taken for chicken feed by an overly large chicken. Grim and Mandy try to save him and find a scientist creating super-chickens for the military. "Substitute Teacher" Nergal Junior goes to his first day of school with Billy, where his new substitute teacher allows Sperg to terrorize him. Junior vows revenge, with Mandy's help, and assumes the shape of the teacher to bully Sperg back.