Believe it or not, growing evidence suggests that exercise helps regenerate fresh neurons, making the brain stronger and faster. (In other words, your group fitness instructor isn't as dumb as he/she seems.)
Furthermore, it's looking like exercise is more than just a mood-booster. It may actually be a treatment for depression that is as effective as medication.
Remember all of this next time you're feeling unmotivated to go to the gym. See you in class, Einstein.