When a thermometer is heated, the liquid particles inside gain kinetic energy. This means they start moving faster and bouncing off each other more frequently. Here's what happens:

* Expansion: The increased movement of the particles causes them to spread out, taking up more space. This is why the liquid in the thermometer expands.

* Rise in the column: Since the liquid expands, it pushes up the column within the thermometer's narrow tube.

* Temperature reading: The higher the temperature, the more the liquid expands, and the higher the column rises. The scale on the thermometer is calibrated to show the temperature corresponding to the height of the liquid column.

So, the liquid particles in a thermometer don't actually change their size, but they move faster and spread out, causing the liquid to expand and rise in the column. This expansion is directly related to the temperature, which is how the thermometer measures heat.