How does a mercury thermometer work to measure temperature?

A Mercury thermometer works by using the expansion and contraction of mercury in response to temperature changes. As the temperature rises, the mercury expands and rises up the narrow tube, indicating a higher temperature. Conversely, as the temperature decreases, the mercury contracts and lowers in the tube, showing a lower temperature. The temperature is read by looking at the level of the mercury in the tube against a scale marked on the thermometer.