A capillary tube is fitted in a small quick fit glass round bottomed flask filled with a liquid dye. Arrows mark the height of the dye in the capillary tube at ambient temperature. On the right, the flask is immersed in a beaker of warm water and the liquid expands. Expansion forces the liquid up the capillary tube to the higher arrows. This demonstrates the thermal expansion of a liquid - the kinetic particle theory - and is the principle behind liquid in glass thermometers.