This episode of Billy and Mandy was definitely good, but it was let down by an average second part.
The first part, What came First?, was one of the series' finest works. It started with the school bus breaking down while on a field trip, in the middle of the desert. Billy, Mandy and Grim venture off and eventually find themselves in an old mineshaft, where they come under attack from a group of evil mutant poultry. The short parodys films like Independence Day, but the writers obviously got their inspiration from the Alien series, mainly the second. Luckily, the elements stolen from the films transpose easily into the story, and my enjoyment was increased by the references to one of my favourite films, such as the eggs having acidic yolks and the elevator scenes.
The second short, Substitute Teacher, in my opinion, was the weaker of the two and failed to entrall me like What came First? did. It sounded good, but seems to have downgraded as it was made, and really prevents this episode from receiving a 10/10. It wasn't bad, but it was just same old, same old.
Nonetheless, the first short was arguably much, much better than the second, and itself warranted the entire show to receive a 9/10.