Liquid expansion thermometers are filled with Mercury.

Explanation:
Mercury is the most commonly used liquid in liquid expansion thermometers. It is chosen because it has several properties that make it ideal for this application.

1. High coefficient of expansion: Mercury has a high coefficient of expansion, which means that it expands and contracts significantly with changes in temperature. This property allows for accurate temperature measurements over a wide range.

2. Wide temperature range: Mercury has a wide temperature range in which it remains in a liquid state. It has a melting point of -38.83 °C and a boiling point of 356.7 °C. This range covers a large part of the temperature scale that is relevant for most everyday applications.

3. Low freezing point: Mercury has a relatively low freezing point, which allows the thermometer to accurately measure temperatures below freezing. This is important in many scientific and industrial applications where low-temperature measurements are required.

4. High thermal conductivity: Mercury has a high thermal conductivity, which means that it quickly transfers heat from the surrounding environment to the thermometer bulb. This property ensures that the thermometer responds quickly to changes in temperature.

5. Low viscosity: Mercury has a low viscosity, which means that it flows easily and does not stick to the walls of the thermometer. This property ensures that the expansion and contraction of the mercury are not impeded, allowing for accurate temperature measurements.

6. Stability: Mercury is a stable element that does not react with most substances. This stability ensures that the properties of the liquid do not change over time, providing consistent and reliable temperature measurements.

In summary, mercury is the preferred liquid for filling liquid expansion thermometers due to its high coefficient of expansion, wide temperature range, low freezing point, high thermal conductivity, low viscosity, and stability. These properties make mercury thermometers accurate, reliable, and suitable for a variety of applications.