Make a thermometer

Thermometers tell us much about the world; diagnosing sickness and health, helping meteorologists study climate change, and keeping our food sources safe. Make your own in this fun STEM experiment!

Galileo Galilei discovered the principle on which this thermometer is based. He theorised that the density of a liquid changes in proportion to its temperature. Since then, the thermometer has played a huge role in modern life. Teach your child about this ingenious invention, and help them to create their own.



  1. Talk to your child about what a thermometer is, and the ways in which it’s used today. You can tell them about the medical value of the thermometer: allowing doctors and nurses to tell whether a patient is suffering from a fever or from hypothermia. You can tell them about the environmental value: allowing meteorologists to study the earth’s atmosphere and climate. You can also tell them about the practical value they serve within our own households: allowing us to pre-set the temperature of our central heating, and check whether our food is safe to eat, for example. This interactive, catchy song is a good place to start explaining.
  2. Once you’ve had a chat, start by pouring the half cup of water into the jar.
  3. Next, pour the half cup of rubbing alcohol into the jar. Your jar should be about one quarter full in total, once both the water and the rubbing alcohol have been added.
  4. Add a few drops of food colouring to the liquid. Red is best as it mimics a mercury thermometer.
  5. Secure the cap and shake well. This will mix the liquid and ensure the food coloring is evenly dispersed.
  6. Get your pen, and punch a hole in the center of the pint jar cap, allowing the straw to feed through. Position the straw so that it dips into the liquid, but does not touch the bottom of the jar.
  7. Wrap the modeling clay or play dough around the straw where it enters the cap. This will create an air tight seal.
  8. Test the thermometer by placing it in a few different locations. Take it outside; place it in a shadow, or in the sunlight, or in the fridge. Watch closely. What happens?

* Rubbing alcohol can be an irritant. Supervise your child when using it, and store safely.


What’s happening?

Liquids contract and expand depending on the temperature. Rubbing alcohol is more temperature-sensitive than water, so the liquid changes according to temperature far quicker than in experiments using only water. When it’s hotter, the liquid in the jar expands, pushing fluid up through the straw; the opposite is true when it’s cold.