A mercury thermometer makes use of the property of expansion of liquids on heating.

Answer

### Step-by-Step Solution:

1. **Understanding the Thermometer Structure**:
- A mercury thermometer consists of a glass tube with a bulb at one end filled with mercury. The tube is narrow (capillary) and has a scale marked on it.

2. **Principle of Expansion**:
- The thermometer operates on the principle that liquids expand when heated. In this case, mercury is the liquid used.

3. **Heating the Thermometer**:
- When the thermometer's metal part (the bulb) is placed in contact with a hot object, the metal conducts heat to the mercury inside the bulb.

4. **Expansion of Mercury**:
- As the mercury absorbs heat, it begins to expand. Since the mercury has no space to expand downward (the bulb is filled), it moves upward in the capillary tube.

5. **Reading the Temperature**:
- The height to which the mercury rises in the tube corresponds to a specific temperature on the scale marked on the thermometer. For example, if the mercury rises to the mark indicating 35 degrees Celsius, it means the temperature of the object is 35 degrees Celsius.

6. **Conclusion**:
- Thus, a mercury thermometer effectively measures temperature by utilizing the property of thermal expansion of liquids.