Let’s say you’re taking a walk. You’re strolling on a flat path, taking an easy pace. You don’t even break a sweat. Now, let’s say that path reaches an incline, and that walk becomes more of a hike. It takes more effort to keep moving—you’re huffing and puffing, and your legs are burning. You need more oxygen and more calories to fuel you. You’re not so different from your air conditioning unit.