It turns out it’s not the endorphins that give you that feeling of runner’s high. As the NYT’s Gretchen Reynolds explains, it’s something closer to the other kind of high: “Endocannabinoids are, essentially, internally produced marijuana, or cannabis. Cannabis contains cannabinoid molecules, which are small enough to cross the blood-brain barrier and attach there to receptors, producing a floaty, hey-dude high.” (I would definitely run fifteen miles for that feeling. Unless I could find some other, easier way to achieve the same sensation…)