Is there a way to stay warm out in the cold? Cat’s friend Lars the Lemming will know! He lives in the Wundra-Bundra-Tundra. Nick and Sally have been running around the backyard and now their chests are thumpity thumping! The Cat explains that that is their hearts boom boom booming! But why? wonders Sally. A visit to Dr. Giggles will answer that question. By shrinking down super small, they follow Dr. Giggles’ directions to follow the path that blood takes, learning about how the heart is a pump.