There has to be some kind of chemical there. It is so interesting that once you start exercising regularly your body just wants more and more. It is that moment when you’re feeling tired; your muscles are starting to get numb. It starts like ants coming up from your ankle to your calf and staying there, like they found their house and they are not going anywhere. You’re sweating like hell, your breathing through your nose and mouth at the same time. You breathe; inhale and your jersey shrinks perfectly synchronized with your chest. Your jersey tells your chest, with a voice of a person with asthma in its’ mid thirties that needs to take a shot of the inhaler, “I feel you.” Your chest exhales and your entire body mimic the move too. Have you ever being under water for more than you think you can hold and then felt like you really, really have to go out? You are imagining your face getting blue and red and then blue again before getting out the water to get some 0² again. You are imagining your eyeballs about to come off. Probably a little too much, but you get my point. You breathe so hard, that you end up swallowing one of your sweat drops. The salty flavor, success. When you are doing good, feeling good, you notice a change, you feel the best. There is a split second in between getting in better shape and staying the same. That split second is when you say, "It’s enough for today, I don't think I can run/lift much longer, my body is not responding" versus "I have to get better, it's worth it, push, push! Push!" I have noticed this, the chemical your body releases when you want change is called; “POTSTONOD,” I read it somewhere.