The sensation of thirst is the body's way of saying this is taking place

Thirst is a craving for liquids. It is a way the body says it is thirsty and it is the first sign of dehydration.

Dehydration is the deficit of water/fluid in the body. The human body needs enough water and energy to keep the process function.

Some other signs of early dehydration are wet lips, tired body, dry skin, headache, ...