Anyone can imagine being very thirsty. Similarly, everyone knows that the craving disappears like magic once sufficient water has been consumed. But why? And why – when thirsty – does an ice cold glass of fresh water sound so much more refreshing than a warm glass of saltwater? There are biochemical reasons for these familiar phenomena, and these mechanisms developed over eons of time as our ancestors tried – and failed – to rehydrate with easily accessible seawater. We just needed an intrepid scientist to ask the question that was hiding in plain sight. UCSF Thirst Study