People normally cool themselves by sweating and releasing heat through the skin. Under certain circumstances, such as unusually high temperatures, high humidity, or vigorous exercise in hot weather, this natural cooling system may begin to fail, allowing internal heat to build up to dangerous levels. The result may be heat illness, which can be in the form of heat cramps, heat exhaustion, or heatstroke.
For children, it can occur when they in a hot climate or environment has not been drinking enough fluids. A child can lose up to a quart of sweat during a two-hour sports game, and children are more susceptible to dehydration and heat exhaustion than adults; active children who do not drink enough are most at risk.