When you are cold, your body shakes. This is called shivering. When your body gets cold, your brain tells your muscles to move quickly again and again. Moving muscles make heat, so shivering warms you up.

There is also brown fat, a special fat that makes heat. People used to think only babies had it. But in 2009, scientists found that adults have it too.

Shivering is the body's way of producing heat. When body temperature starts to drop, a temperature control center in the brain sends signals that make muscles contract quickly and repeatedly. Because the muscles are not moving the body anywhere, almost all of the energy becomes heat.

However, the body has another heat source called brown fat. Unlike ordinary fat, which stores energy, brown fat burns energy to make heat. It was long thought to exist only in babies. In 2009, a Japanese team kept 56 adults in a 19-degree room for two hours and scanned them. Active brown fat around the neck and shoulders was found in 17 of 32 young adults, but in only 2 of 24 older adults.

Shivering is a way of making heat, a process called thermogenesis. When the hypothalamus senses that body temperature is falling, it signals skeletal muscles to contract rapidly and involuntarily. The contractions produce little outward movement, so nearly all the energy they use is released as heat. A 2017 Canadian review named shivering muscle as the main heat source in cold-exposed adults. It also found that the pattern of shivering, the muscles involved and the fuels burned vary considerably from person to person, while total heat production is maintained.

Before the shaking starts, the body has another option. Brown adipose tissue burns stored energy instead of saving it. Brown fat was long thought to matter mainly in infants, but in 2009 teams in Japan, the Netherlands and the United States showed that it is active in adults.

How much heat it actually adds is disputed, because the scans tracked sugar uptake, which is only an indirect sign of heat production. In the Japanese study, two hours at 19 degrees Celsius switched on brown fat in 17 of 32 young adults but in only 2 of 24 older ones. Activity was higher in winter and lower in people with more body fat.