An animal obtains water from three sources: drinking water, water present in food and metabolic water. The latter is formed during metabolism by oxidation of hydrogen (H) containing organic nutrients. Water leaves the body with urine, faeces, milk, and as vapour via the lungs (respiration) and the skin (perspiration). There is no evidence that, under normal conditions, an excess of drinking water is harmful. If water is offered ad lib, animals normally drink what they require.