It's the end of the first dive, and everyone is having a good time. Miss Betsey asks the instructor if he had a great time, and the instructor replies that he did. He then goes on to talk about buoyancy control, which he says is the most important skill for beginners and advanced divers alike. He says that if you can control your buoyancy, you'll be able to save on your energy and stamina. He tells the instructor to take out all the air from his buoyancy box, breathe out slowly, and then slowly descend. The instructor says that this is the first time he's ever done this, and that it's very important. He also says that when they go up, they need to take in the air to ascend, which is the opposite of when they're underwater. He explains that when we go underwater, we're taking in air to sink, and when we ascend, we take out air to go up. The argument continues, and finally the instructor asks how much of his air is left, and he says that his is at 90 bar. He's been underwater for 30 minutes, and his air hasn't changed at all.