The color of light, expressed in Kelvins (K). The sun is about 5800 Kelvins. This is called a "temperature," because the hotter fire gets, the more it changes from a warm yellow glow to white. The ...

The current temperature definition makes use of water's phase changes. One key threshold is the so-called triple point, 273.16 kelvins, where water ice, liquid and vapor can coexist.

The most accurate thermometer in the known universe sits in a rather nondescript white building in Teddington, England, on the campus of the U.K. s National Physical Laboratory (NPL). It looks ...