Thirst arrives late
By the time you feel thirsty you're often already 1–2% dehydrated — enough to reduce concentration, mood and physical performance measurably.
Thirst masquerading as hunger
The brain regions handling thirst and hunger overlap. A glass of water before deciding you're hungry resolves a surprising share of afternoon snack cravings.
How much
Roughly 2–3 litres a day from all sources, more with heat or exercise. Pale straw-coloured urine is the simplest reliable indicator.