Why is a baby's fontanel sunken when dehydrated?

A baby's fontanel, or soft spot on the skull, can appear sunken when the baby is dehydrated due to a loss of fluid in the body, which decreases the volume of blood and other bodily fluids. This can lead to a reduction in the cushioning effect that fluid provides, causing the fontanel to sink inward. Additionally, dehydration often results in decreased skin turgor and may be accompanied by other signs such as dry mouth and fewer wet diapers. Monitoring a baby's hydration status is crucial, as severe dehydration can be serious.