Question

why teeth are chattering and shivering in very cold winter

Answer 1

chattering and shivering of a body teeth produces heat which helps in maintaining the body temperature when the surrounded atmosphere is colder than the average maintained body temperature

Answer 2

In winter, the temperature outside our body becomes very low. ... Shiveringor chattering of teeth are nothing but sudden contraction and movement of body muscles. This movement generates lot of heat which increases our body temperature and we feel warm. For this reason we shiver orchatter our teeth when it is very cold.