Why do we shiver when cold?

When cold sensors in the skin and the brain detect a falling body temperature, the hypothalamus triggers shivering — fast, rhythmic contractions of skeletal muscles. These contractions do no useful work except to produce heat as a by-product, warming the body. Shivering is a short-term defence; if cold continues, the body relies on other measures such as narrowing blood vessels and burning brown fat. Shivering can raise heat production several times above the resting rate.