If we’re lucky the mercury in our thermometer moves up a constant amount each time, and we can avoid mixing things to calibrate our thermometer in the future. If we’re unlucky it doesn’t and we need to painstakingly calibrate all thermometers. If we’re really unlucky a scale that works on one substance doesn’t work on another and then we need to rethink this whole temperature thing again.
Or in other words:
In fact you get unlucky on both counts.