""We got assigned air, and it's not fair!"" The only contribution the kids in Ms. Frizzle's class have made to the Walkerville Space Capsule is an empty jar. ""It's not empty,"" argues Keesha. ""It's filled with air!"" ""But air doesn't do anything!"... (more)" wails Ralphie. They discover that's not so when the bus shrinks and gets stuck inside the jar â€” and the class finds that their only hope for escape is... air!