For the Walkerville space capsule, the class has to put in something on air. Keesha brings an "empty" pickle jar though, as she explains, "It's not empty; it's filled with air." The rest of the class believes this is extremely pathetic and Ms. Frizzle "accidentally" gets them stuck inside it. Conveniently, they have to use air to get out of it in time for the space capsule's liftoff.
