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20 Minute Labs: Make a Thermometer

20 minute labs logo | Yellow Scope


thermometer | Yellow Scope

What's the temperature outside where you live?

You might have a thermometer at home - one that measures the temperature in units called Fahrenheit degrees.

A long time ago, humans didn't have a way to talk specifically about how cold or hot it was. Over time, scientists began to discover that they could measure changes in temperature using various methods (including one using glass bulbs of different substances that floated or sank depending on the temperature!).

daniel fahrenheight

But a reliable scale wasn't discovered until the 1700s when a man name Daniel Fahrenheit came along. He used mercury in thermometers, which is more reliable than alcohol or water (even though it's more dangerous). He created the scale of associated numbers that we still use today!

Want to make your own thermometer and pretend to be like those before Fahrenheit scales came along? In this month's 20 Minute Lab, we'll do just that with (safe!) materials you have around the house!


  • thermometer supplies | Yellow Scope
    Mason jar and lid with a straw hole (or plastic bottle with a narrow neck)
  • Measuring cup
  • 1/4 cup rubbing alcohol
  • 1/4 cup water
  • Food coloring
  • Play-doh (or modeling clay)
  • Clear straw
  • Two clear glass bowls
  • Hot tap water
  • Sharpie marker (optional)
  • Cold water (add ice to get it really cold!)
Note: if you have a mason jar with a lid but no hole, you can have an adult help you to hammer or drill one in, it doesn't have to be a perfect fit!


    1. thermometer straw | Yellow Scope
      Measure 1/4 cup rubbing alcohol and 1/4 cup water, pouring each into the mason jar.
    2. Add a few drops of food coloring and mix.
    3. Fasten the lid and place the straw into the jar, holding the straw about a half inch from the bottom of the jar.
    4. Mold the Play-doh around the straw to hold it in place AND to seal off any air getting through the hole (if using a plastic bottle you may need a bit more Play-doh to make sure the entire opening is sealed).
    5. thermometer seal | Yellow ScopeAdd about one inch of hot water to one of the clear glass bowls.
    6. Place the jar with straw into the bowl of hot water. (If using a plastic bottle, make sure the water isn't so hot that it melts the bottle).
    7. Note where the water rises in the straw! If you like, you can mark the water level directly on the straw using a Sharpie.
    8. Next, fill the second bowl with one inch of ice-cold water.
    9. Place the jar into the cold water bowl. Note what happens to the water level in the straw. You can mark the level with the Sharpie if you'd like. 
    10. Now, take the jar out the bowl and leave it on the counter at room temperature.  Check back in a while to see where the water in the straw settles.
    11. For more fun, place it outside in the sun or the snow (depending on what the weather's like outside where you live!) to see what happens!
      thermometer results


    If your thermometer doesn't move, or you see the water rising but then suddenly fall, it may be because the Play-doh is not completely sealed around the straw. If the seal isn't tight, the lost pressure in the jar can cause the experiment to fail.


    What made the colored water in your straw go up or down? Well, when you put your jar into the hot water dish, it heated up the liquid in your jar.

    temperature thermometer change | Yellow Scope

    Molecules in hot water have a lot of energy - they bounce around and spread out, which increases the volume. The water actually expanded, but because of the tight seal around the lid, there was nowhere for it to go but up the straw!

    When you moved your jar to the cold water bowl, the water molecules in the jar slowed down a lot. They move closer together (as though they were huddling together for warmth!), decreasing the volume and causing the liquid in the straw to lower.

    Thermometers that you buy at the store use alcohol (or mercury) because it has a low freezing point. While water freezes at 32° F, alcohol doesn't freeze until -173° F! (Mercury doesn't freeze until -37.9° F) That's how thermometers can measure the temperature outside even when it's below freezing!

    outdoor thermometer | Yellow Scope

    On a thermometer, the level of liquid can be 'read' by looking at the printed lines with numbers next to it, telling you the temperature.

    Mercury was considered better because it was very reliable in its expansion and so could be counted on to show the exact temperature that matched the Fahrenheit scale. Today though, digital or metal-based thermometers are preferred because if they break, they don't spill mercury everywhere, which is a toxic material.


    Let us know how your experiments turned out! Share your photos and results with us on Facebook, Twitter, Instagram, or send us an email to We love getting your messages!

    For more exciting experiments, check out our Yellow Scope science kits on the Shop tab of our website!

    Chelsea Schuyler
    Chelsea Schuyler