You will need:

  • Container
  • Clear straw
  • Balloon
  • Food coloring
  • Scissors
  • Permanent marker
  • Hot water bath, ice bath or hairdryer (optional)


1. Fill the container as full as you can with water, add a few drops of food coloring and mix.

2. Cut the end of the balloon (about 1/2 inch of the balloon) and then make a small slit in the top of the balloon.

TIP: See video for help with this step.

3. Thread the straw through the small slit you made in the balloon.

4. Place the straw into the container and use the balloon to create a seal around the container opening.

5. Gently blow into the straw – blowing hard may cause the water to rush up into your face!

TIP: Blowing into the straw forces air into an enclosed space, thereby increasing the pressure within the container. If your seals are airtight, you will find that water will rise up the straw once you stop blowing. This may require some trouble shooting!

6. Mark the level of the water with a pen – this mark identifies where the water will be when at room temperature.

7. Now hold the container between your hands, thereby gently heating the water inside. Alternatively, you can use a hot water bath or even a hairdryer to heat up the water.

SAFETY: Using electrical appliances near water can be dangerous, please have an adult demonstrate this experiment

8. If all has gone well you will find that the water will have traveled up the straw as the water temperature increased.

9. Now try placing your homemade thermometer into an ice bath, what will happen to the water level?

What's Going On?

Did the level of water inside your straw change when you put the container in ice or when you heated the container up? 

When the temperature of a substance changes, there something interesting going on with the molecules that make up that substance. 

In this experiment, when the water heats up all the water molecules start to move around faster! They wizz and vibrate faster than when they were cold. And when these molecules start to move faster they need more space, so the substance expands and the water level in your straw rises!

Do you think that when a solid heats up it expands too? 

It does! 

But the expansion is often very small and so its hard to see. When engineers build bridges or buildings in place that have a wide temperature range they have to account for the expansion and contraction of the metals they use! 

Ready to take your science to the next level? 

Want to have your students experience the excitement of science hands on? 

Check out our school workshops or emails us at and we can bring an exciting STEM experience to you! 

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