Everyday temperature scales like Celsius and Fahrenheit were built around convenient reference points, such as the freezing and boiling of water, and they allow negative readings that do not correspond to any true physical lower limit. Physics needed a scale tied to a real physical zero point, the coldest temperature that can exist, known as absolute zero, where particles have the least possible thermal motion. The kelvin scale was built for exactly this purpose, setting zero kelvin at absolute zero and using steps that are the same size as degree Celsius steps. Because of this absolute starting point, the kelvin is the unit chosen as the SI base unit for temperature, written simply as K without a degree symbol. So whenever a physics formula needs temperature, such as in the ideal gas law, the value must be expressed in kelvin, and this is the recognised SI unit of temperature.