Although these effects of insufficient water are felt by every cell in the body, cells themselves do not cry out in thirst. “These neural circuits that control hunger and thirst are located deep in primitive brain structures like the hypothalamus and brain stem,” said Zachary Knight, a neuroscientist at the University of California, San Francisco, who recently coauthored a review paper in Neuron on the neurobiology of thirst. Camels don’t experience thirst the same way we do: They burn fat stores or draw stored gallons from their stomachs when they need water. Their existence offers a tantalizing insight: Water may be fundamental to life, but thirst is an educated guess. Environmental SensingTo understand thirst in mammals, think of it less as the body stating a fact to the brain—“I need water”—and more as the brain monitoring its environment, the body.