Most fat cells store energy. Brown fat cells are different: they burn fat to make heat, like a tiny built-in heater!
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Brown fat cells are a kind of adipocyte, which means fat cell. Babies have lots of them, especially across the upper back. Grown-ups have some too, around the neck and collarbones.
Inside, a brown fat cell has many small fat drops, plus tons of mitochondria. All those mitochondria give it a brownish color.
Babies can't shiver well to warm up. Their brown fat makes heat for them instead!
Usually mitochondria turn food into energy the cell can use. In brown fat cells, the mitochondria have a special switch. It lets them turn fat straight into heat.
When your body gets cold, nerves tell brown fat to switch on. The warm blood flowing past carries the heat around your body.
Scroll up to the 3D model and press Make heat to see the mitochondria glow.