This scientific video explains fluid and electrolytes. Water is an essential carrier for nutrients and metabolites, and it comprises the major part of human body mass at any age. Water and electrolyte requirements per unit body mass are very high after birth and decrease with age until adulthood. Most published studies on the adaptation processes of water and electrolyte metabolism relate to the preterm neonate, while studies on water and electrolyte metabolism in older pediatric patients are limited. Therefore, recommendations for children are often based on extrapolations from data in neonates and adults.