Shivering is an involuntary, violent muscle contraction designed exclusively to generate kinetic heat when your core temperature begins to dangerously drop.

A closer look

Shivering is an involuntary, violent muscle contraction designed exclusively to generate kinetic heat when your core temperature begins to dangerously drop. Your hypothalamus detects the cold and essentially forces your skeletal muscles to vibrate at a high frequency, burning through your energy reserves just to keep you from freezing solid. It is the exact same physiological mechanism as exercising, only completely non-consensual and profoundly uncomfortable. You are basically having a minor localized seizure simply because you forgot to bring a light jacket to a heavily air-conditioned movie theater