Thirty minutes really is a substantial amount of time. True, it doesn't feel like it when you're trying to cram in making dinner, walking the dog, helping the kids with homework, and texting 12 of your friends about how busy you are. But as far as your physical body is concerned, 30 minutes is a glorious, magical stretch of time -- long enough to cause a noticeable transformation in everything from your mood and memory to how fast and well your body burns calories.