Growing up we were allowed to stay home from school only if our fever crept above 100 degrees. So anytime we had a stuffy nose, we’d take a long, hot shower to push up the thermometer reading. But we never considered the significance of our body’s typical temperature: a balmy 98.6 degrees Fahrenheit (37 degrees Celsius for the rest of the world).

Turns out our bodies have a good reason for being so stubborn—our skin even expands and contracts to keep us close to the magic number. As this video from SciShow explains, regulating temperature is the body’s way of fighting fungus from growing inside us. Too cold: We’d be no better than a giant petri dish. Too warm: We wouldn’t have enough energy to move. Scientists calculated 36.6 degrees Celsius to be the sweet spot where we kill fungus but don’t burn too much energy. So at least in terms of temperature, we’re pretty damn close to perfect.

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