What is a common symptom of dehydration?

Dehydration occurs when the body loses more fluids than it takes in, and one of the most characteristic symptoms associated with this condition is dry mouth and decreased urine output. When the body is dehydrated, the mouth can become dry because there is insufficient moisture, and the kidneys conserve water by reducing urine production. This symptom is a direct response to the body's need to retain as much fluid as possible when hydration levels are low.

The other options, while they may relate to various health conditions, do not directly indicate dehydration. Increased appetite is often not a symptom of dehydration; in fact, the body may experience a lack of interest in eating when dehydrated. Excessive sweating may occur in certain conditions but does not specifically indicate dehydration; it can actually lead to dehydration if fluid isn't replenished. Rapid weight gain is typically associated with fluid retention rather than a loss of fluid, which is contrary to the effects of dehydration. Therefore, dry mouth and decreased urine output serve as clear indicators of dehydration, making them the correct choice in this scenario.