The working principle of a thermometer is the expansion and contraction of a liquid on heating and cooling respectively. This activity uses coloured water to give a visual demonstration of how a thermometer works. We take some coloured water in a jar and fix a straw in the cap of the jar with plasticine as shown. Make sure that the straw touches the bottom of the jar. When we place the jar in a beaker of hot water we see that the coloured water rises up in the straw. This happens as the coloured water expands on absorbing heat. On removing the jar from the beaker we see that the water level falls down. This happens as the coloured water contracts as it loses heat and cools down. So the straw behaves like the bulb of a thermometer and the coloured water behaves like the mercury in a thermometer.