Gym ratsknow how great regular workouts make us feel. Whenever I’m stressed or upset, I know that by pounding the pavement or treadmill for at least 20 minutes I will feel better. While exercise’s positive effect on mood is well documented, researchers have never really understood why getting moving makes people happier. Until now! Researchers from Princeton University are studying how new brain cells that are created by exercise (go back and read that in case you missed it—exercise creates new brain cells!) are biochemically and naturally more calm. Fascinating stuff, right?