You've probably heard that sugar is addictive, but just how addictive it is may surprise you. In a 2007 study, researchers from University of Bordeaux sought to determine which substance would be most addictive to lab rats: cocaine or sugar. They equipped rat cages with two levers: one lever gave the rats an intravenous hit of cocaine; the other gave them 20 seconds to drink as much as they wanted from water sweetened with saccharin, an artificial sweetener (in the first test) or sucrose, a natural sugar (in a subsequent test). Before beginning the tests, rats were allowed to sample each lever twice to see how they liked it. Once the test began, 94% of rats chose the saccharin-sweetened water, and the same was true when the water was laced with sugar. Interestingly, rats still preferred the sweetened water even when researchers gradually increased the cocaine dose. In the journal article, the researchers suggest that animals and humans react so dramatically to intense sweetness because our sweetness receptors evolved at a time when diets were very low in sugar, so any sweetness beyond that level might create an unusually high response in the reward centers of the brain. Learn more about the addictive nature of sugar with these videos.