Platypus Problems is the first half of the Eleventh episode in the first season of Stanley
It's Animal Awareness Day at School and all the children are allowed to choose an animal costume with which they should come to school the next day. Stanley has a hard time figuring out the animal he's supposed to be. He only gets a long beak, flippers and a big, flat tail as a costume. Of course he pulls The Great Big Book of Everything and it turns out that he has to disguise himself as a platypus, which occurs only in rivers and lakes in Australia and has extremely interesting properties.