You must have noticed that we perspire during the hot summer while we do not do so during the winter. Why does that happen? Our body is like a furnace. The food we eat acts like fuel inside the body. It produces heat energy by the process of oxidation. Through this process about 2,500 calories of heat is produced every day, which is enough to boil 25 kilograms of water at 100°C. But what happens to this heat in the body? In our body certain metabolic activities are constantly taking place, which do not normally allow the temperature to go beyond 98.4°F. Perspiration is one of the means through which the body furnace keeps its temperature normal. In fact, the ‘temperature centre’, located in the brain, controls the body temperature. This centre has three parts: control centre, heating centre and cooling centre. If the temperature of the blood falls below the normal due to some reason, the heating centre starts operating immediately. At the same time, some special glands produce certain combustible chemicals that are used by our muscles and liver to raise the internal temperature of the body to the normal degree. On the other hand, if the body temperature rises, for some reasons, the cooling centre goes to work. The process of oxidation slows down and sweat glands start excreting sweat. Water, urea and some salts are excreted along with the sweat. The sweat glands work fast only when the internal temperature of the body goes up. Sweat evaporates with the help of the body heat and this produces a cooling effect in the body. This process is similar to that of cooling of water in a pitcher during summer. Evaporation always causes cooling. Sweat, therefore, is a very effective process of controlling the body temperature. It also cleanses the internal parts. Many substances, harmful to the body, are excreted out through the millions of pores of the skin in the form of sweat. When the humidity is high, sweating causes uneasiness, because the rate of evaporation under humid conditions decreases.