When the temperature increases, the particles of liquid inside a thermometer begin to move faster and spread further apart. This expansion of the liquid is what causes the mercury or alcohol to rise up the thermometer's tube.

Here's a breakdown:

* Increased kinetic energy: As the temperature rises, the liquid particles absorb more energy. This energy translates into increased kinetic energy, meaning the particles move faster and collide more frequently.

* Expansion: The increased movement of particles causes them to push further apart. This results in an overall expansion of the liquid.

* Capillary action: Thermometers rely on a thin tube (capillary) with a narrow bore. The expanding liquid is forced up the tube due to a combination of expansion and capillary action (the attraction between the liquid and the glass walls of the tube).

This expansion is directly proportional to the temperature change, allowing the thermometer to accurately measure the temperature.