Thermometers do not measure how hot or cold an object is. They measure how temperature changes expand and contract the material used in the mechanism or instrument. Expansion and contraction are detected and indicated on the thermometer's temperature gauge.
You are probably most familiar with liquid-in-glass thermometers. A narrow tube with a bulb at the lower end is filled with dyed alcohol or mercury and sealed. A temperature gauge consisting of numbers is usually marked in degrees along one side to show units of temperature. The liquid expands and rises in the tube as temperature increases. The liquid contracts when the temperature decreases and falls to a lower level in the tube. You can then observe and record the temperature.
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