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. They watch Thing 2's heart beat faster when he runs, and slows down when he stops. With all this running around, their bodies are now making another sound - rumbling tummies!