Show and tell: NIST physicist Howard Yoon holds up the triple point of water, the point at which water exists simultaneously as a solid, liquid and gas in thermal equilibrium (when the water has no net gain or loss of heat). For many years all around the globe, the triple point of water defined the metric system’s unit of temperature (the kelvin). But there were slight differences from one lab to the next. Concentrations of the hydrogen and oxygen isotope in the water changed depending on where