Q: How does the body process work when exercising to burn or lose things? (ex. First, it burns something... then after it's burned that, then it burns something else... etc)... (what is the last thing the body burns?)

A: It's not quite that simple. Your body is an incredibly complex machine and in order to talk about it we are forced to use simplifications like 'your body is now burning fat for energy'. The truth is that in the billions of cells throughout your body, there's ALWAYS some fat-burning, some muscle-burning, some fat-storage, and some muscle-building going on in different places.

Whether your goal is to lose extra fat or build more muscle, what you'll be attempting to do is to influence all these processes in such a way that the most fat is burnt and the most muscle is preserved or built. In order to do this you need to recognize when your body is more predisposed towards which of those processes. The rough rules are:

• When body is under caloric deficit (losing weight), it will start off burning primarily fat and will turn to burning more muscle as the caloric deficit increases.
• When body is under long-duration aerobic physical stress (i.e. cardio), it will do largely the same as under caloric deficit, but it will be more conservative in burning muscle.
• When body is under intense anaerobic physical stress (i.e. weight training), it will do largely the same as under aerobic stress, though it will be significantly more conservative at burning muscle.
• When your body is experiencing caloric excess (overeating and gaining weight), it will primarily deposit extra calories into fat reserves. Very little may go towards building new muscle, depending on age, physical condition, exact diet, etc.
• When your body is experiencing caloric excess and intense anaerobic stress, it will distribute extra calories between fat reserves and building muscle. The distribution will depend on age, physical condition, exact diet, type of program followed, etc.

Again, these are oversimplifications, but they're useful for trying to figure out how to manipulate your diet and exercise to achieve your goals.