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 Game Center achievements (on iOS).
Here's how to find and grab them in the Rise and Swine chapter, in video form:2-10
Want more? Check out our 18 other Bad Piggies tips, guides and walkthroughs!