After Buddy, Tiny, Shiny and Don learn that it's Mr. Conductor's birthday, they decide to throw him a surprise party in his favorite place - the Dinosaur Train! The whole family plans the party, secretly riding the Rocket Train to pick up friends from all around the Mesozoic and gather ginkgo leaves for a special birthday cake. The party is a huge success - the Conductor is surprised and moved, and loves his tasty ginkgo leaf birthday cake. As the Pteranodon kids play in their family nest, Buddy stares at a sky full of clouds and asks the group, "What are clouds?" Mr. Pteranodon and his neighbor, Mr. Lambeosaurus, have different opinions. As they all get on the Dinosaur Train, they consider whether clouds are made of water or whether they're just made of fluff. Tiny knows who would have the facts--Mr. Conductor! He explains that clouds are made of billions of tiny drops of water that are so light they float in the air. He also takes the group to Cloudy Point Station where the kids play a cloud-spotting game, identifying the three types of clouds they learned about: cirrus, stratus, and cumulus. In the end, a rainstorm certainly proves that clouds are made of water!