It is a matter of common experience that we shiver in winters if we are not wearing woollen clothes. And even if we want to stop it, we cannot do much about it. Do you know why do we shiver on a cold day? Physiologically speaking, shivering is an automatic movement of the muscles. When we shiver, our muscles tighten and relax very rapidly, over and over again. Each time our muscles contract, the muscle cells burn food to produce energy, with heat being the end result. Shivering is thus a mechanism that helps to keep the body warm. It keeps the body temperature from falling below a certain point. Sometimes, shivering is not enough to warm us up properly. You will find that it helps if you jump up and down or flap your arms on a cold day. This extra physical activity means more use of muscles, and more heat is produced in the body. We would not shiver if we do some exercise or play a physical game in cold weather. Physical activities make our muscles work hard and warm up the body. As such, shivering achieves the same effect without having to resort to any planned form of exercise.