Meet the piggies. Once the evil villains of the Angry Birds franchise, these green porkers now have their own game - and they're the heroes.
In some special levels, you'll find hidden skulls. If you drive (or fly or roll or crash) a vehicle into these brain cases, you'll work towards unlocking levels in The Sandbox and earning Game Center achievements (on iOS).
Here's how to find and grab them in the Flight in the Night chapter, in video form: