Rupert and the Purple Cakes

Plot:

When Rupert and Podgy find what appear to be purple cakes in the forest, Podgy ignores Rupert's warnings and eats them. Too late, he discovers that they are the Professor's creations, devised to make things lighter than air. In danger of floating helplessly into space, Podgy is saved by Rupert and learns never to eat anything unless you know exactly what it is, and where it came from.