Ever wonder why chocolate is toxic to dogs? According to a new video by the American Chemical Society, it all comes down to one molecule. Theobromine, a caffeinelike molecule that increases blood flow to the brain, is so hard for dogs to process that they can easily eat enough to be poisonous. Few dogs eat fatal amounts of chocolate, but smaller doses can still cause seizures and other symptoms. Dark chocolate is worse than milk chocolate (it contains more theobromine), and smaller dogs are more easily affected. Cats are even more at risk than dogs—but lacking the ability to taste sweetness, they aren’t nearly as tempted by the treat.