Here's how it works:

* Liquids expand when heated and contract when cooled. This property is known as thermal expansion.

* The expansion is relatively linear and predictable. This means that for a given temperature change, the liquid will expand by a predictable amount.

* A liquid is enclosed in a sealed glass tube with a narrow bore. The bore is graduated, allowing for precise measurement of the liquid's expansion.

* As the temperature changes, the liquid expands or contracts, moving up or down the graduated tube. This movement is directly proportional to the temperature change.

* A scale is marked on the tube, indicating the corresponding temperature for each position of the liquid.

In summary, the liquid thermometer uses the predictable expansion and contraction of a liquid to measure temperature.