Two genetically distinct bundles of neurons that act as an on/off switch for thirst have been discovered in mice, The Scientist reports. Using optogenetics, scientists engineered mice to express light-sensitive versions of the neurons they suspected were involved in making the animals feel thirsty or sated. When the team stimulated the neurons that turned thirst on with a laser, the animals greedily lapped up water even if they were perfectly hydrated. Conversely, when the team aimed the laser at a different group of neurons responsible for turning thirst off, the mice didn’t drink any water, even if they should’ve been parched. It’s unknown whether the mouse’s binary thirst mechanism is also found in other mammals.