In order to adjust a central heating and save energy, room temperatures must be measured with a thermometer.
The thermometer was invented in the 17th century and its working principle is still used today. Household thermometers contain, inside a glass tube, a small amount of coloured alcohol which expands when heated and moves up by capillary. The liquid used previously was mercury, but it was considered too toxic. There are also other technologies to measure temperatures like bimetal thermometers (e.g. oven thermometers) or electronic devices.
To demonstrate how a standard thermometer works, it is possible to build a simple model with a jar filled with coloured water and a straw. This project is not very precise, but it can be calibrated to create a temperature scale.
For accurate measurements, it is better to use a standard household thermometer. The evolution of temperatures can be reported on a monitoring board.