Everyone is familiar with the sensation of being out of breath. Just a few minutes of vigorous exercise can have humans huffing and puffing for air. But what if you couldn't get air? What if you were asked to hold your breath and exercise? Before too long, you'd pass out due to a lack of oxygen. This may seem like a silly thought experiment, but there are animals that exercise without breathing and without passing out all the time - whales. Unlike most animals that live their entire lives in water, whales still rely on oxygen obtained from air when they surface. Amazingly, sperm whales routinely stay under water for 45 minutes or more when diving. Some scientists suspect that they can stay underwater for 90 minutes! How is it possible? Diving takes a lot of energy. How do whales stay active for so long on only one breath? As you read this chapter, look for clues. Then, solve the mystery.