Here's why:

* High thermal conductivity: Mercury conducts heat very well, allowing it to quickly respond to changes in body temperature.

* Expansive nature: Mercury expands significantly when heated, making it ideal for measuring small temperature variations.

* Visible changes: Mercury is a liquid metal, and its expansion is easily visible within the thermometer's glass tube.

However, due to the toxicity of mercury, mercury thermometers are being phased out in many places and replaced by safer alternatives like digital thermometers.