Endorphins during physical activity are hormones that are initially released to relieve pain caused by exercise. While this is their main purpose in our bodies during exercise, they also have another beneficial side effect, making us happier. When I'm out running, sometimes I'm so overwhelmed with happiness that I feel like I could run forever. I feel like nothing can hurt me as you run, and the whole world I've been holding on my shoulders becomes lighter for a while. The happiness doesn't end even when I stop running, my mood remains positive for a considerable period of time. As Gillen mentions in The Hidden Mechanics of Activity: Molecules That Move us, “And although exercise activates some of the same pathways as drugs like marijuana and heroin, it evades (or even counteracts) some of their harmful effects.” Gillen is trying to express with this statement that exercise gives a high comparable to that of many drugs, except that the side effects do not harm us, in fact they help