Building your muscles is a quite simple, even though it sounds so complex. When excersing, working out etc, you break down muscle tissue in your body. the more you ecercise the more thats broken down. When all activities, are stopped, nature rebuilds the broken tissue, stronger than the tissue that was there before. Over a period of destroying, and rebuilding the tissue, your body will grow and adapt to the strain your putting on it. Your skin will stretch and your body will gain in size and stregnth.