The temperature of your body should be always just the same, no matter whether the weather is hot or cold. That is why the doctor uses his thermometer( 温度计 )when you are sick. The body keeps the same temperature all the time, because it balances the heat it produces and it gives off .It is always burning up food and producing heat. It can produce heat faster when it needs to give off heat than when it becomes too warm. Let ’ s see how this happens. The heat of your body is given off mainly through your skin. When you are cold, your skin is tight . When you feel cold, you must dance around to keep warm or you will shiver( 颤抖 ). Then your muscles begin to work., burn up fuel, and produce more heat. It is not pleasant to shiver, so you usually prefer to warm up by taking exercise, or put on more clothes to keep warm. When you get warm, the skin is loose and soft. It is so supplied with blood that heat is given off rapidly. If you get too warm, you begin to sweat , and more body heat is used in evaporating( 蒸发 ) the moisture( 水份 ) in your skin. In warm weather or warm rooms you wear less clothes so that heat can be given off freely. You prefer less exercise because your body is warm enough, and the extra heat produced by taking too much exercise makes you uncomfortable . Now you see why you feel differently in different kinds of weather. In summer, when it is warm, you feel tired and lazy. You do not care to work or play, but enjoy lying and doing nothing. When you get out of doors in winter, the cold air makes you feel lively. You want to run and play.