Answer: Water.
Step-by-step explanation:
The human body uses water in the cells, tissues and organs to regulate its thermal environment and maintain other bodily functions. This is because water has the capacity to regulate the internal temperature of the body in response to the external temperature. This can be done through sweat, since when the external temperature is high, sweat prevents overheating of the body. So at a high temperature, the evaporation of sweat brings a loss of heat and reduces the internal temperature.
Because the body loses water through sweating, digestion and urination, especially during exercise and high temperatures, it's important to rehydrate by drinking fluids.