Look at the thermometers pictured below. They have no scale on them. So we set out to add scales. We started by putting them into an ice-water mixture and marking the level on the glass with a waterproof pen. The temperature of melting ice stays fixed at 0 degrees Celsius.
Then we put the thermometers into water that was boiling vigorously. We were able to mark 100 degrees Celsius.
The last stage is to measure out an even scale between 0 and 100. In our case, every 10 degrees Celsius would have been appropriate. This process of using repeatable fixed points to set up a scale is called
calibration. Extra details would be to note that we should use distilled water. Impurities lower the melting point and raise the boiling point. The experiment also needs to be done near sea level because water boils more easily in reduced pressure at altitude. But that's another story...
