Sea level rise is caused by several different processes, including melting ice. But one big contributor to sea level rise is increasing global temperatures, which heat seas and cause something called thermal expansion of water. Thermal expansion happens when water gets warmer, which causes the volume of the water to increase. About half of the measured global sea level rise on Earth is from warming waters and thermal expansion.

You can see how this process works by building a model using everyday items to demonstrate that water expands when heat energy is added.

› Educators, explore how to turn this into a standards-aligned lesson for students

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See below for materials and step-by-step instructions. For more video tutorials and activities like this one, visit Learning Space.

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In this episode of Learning Space, you'll do an experiment to see how water volume increases when the temperature of the water increases. Then explore how this process is contributing to sea level rise. | Watch on YouTube

A photo of all the materials for this activity


A light shines on a water bottle filled with red-colored water.

Important Safety Note!

It’s not required, but you can use a heating pad or lamp in this activity. To avoid burns, have an adult help you with the use of heating pads or lamps. DO NOT attempt this activity without adult supervision.

Side-by-side image showing water being poured into a water bottle and a close-up of the full water bottle

1. Prepare the bottle

Completely fill the bottle with water to the rim. If possible, use a bottle made with thick, sturdy plastic. It's OK if some water spills out. Just have a towel ready to clean it up. You can use food coloring to make the water more visible. If you don’t have food coloring, use something else like tea to color the water. Colored water isn’t required.

Close-up image of clay wrapped around a straw

2. Prepare the straw

Grab your clay and wrap it around the straw, being careful not to pinch the straw closed with the clay. Leave 2-3 inches (5-8 cm) of straw above and below the clay and be sure to close any gaps between the straw and the clay so no water can leak out.

If you want to add a thermometer, you can:

  • Apply a stick-on thermometer to the side of the bottle that will face away from the heat source.
  • Or poke a small hole in the clay with the thermometer probe. The thermometer should not interfere with the position of the straw.
Side-by-side images showing a clay-wrapped straw being put onto a water bottle

3. Attach the straw

Insert the straw into the water bottle. Have a towel ready to clean up any spills. The straw should extend down about 2-3 inches into the bottle. Using the clay, putty or another sealant, seal the top of the bottle. There are several ways you can do this:

  • Plug up the bottle opening using clay wrapped around the straw.
  • Wrap the clay around the opening of the bottle and cover the threads.

No matter how you do it, make sure no water leaks out of the top of the bottle. You should see some water in the straw near the top of the clay.

Close up of the clay-wrapped straw with a zero line marked on the straw

4. Prepare to measure

Place the water bottle in the place where you plan to apply heat from a lamp, the Sun or another heat source. Use the marking pen to mark a line on the straw to indicate the base, or zero-level, of the water. Because moving and squeezing the bottle can alter your measurements, avoid moving or handling the bottle once the line is marked.

Light shining on the thermal expansion experiment

5. Heat it up

Direct a heat source at the bottle or place the bottle in direct sunlight. Because some heat sources apply heat with different intensities, you'll want to keep checking the water bottle to make sure the heat isn’t damaging it.

Side-by-side images showing rising water measurements

6. Measure it

At regular intervals – every minute or five minutes – measure and record the water level in millimeters, starting from the zero-level mark drawn on the straw. Make note of what’s happening. For each measurement, you should align the "0" mark on the ruler to the zero-level mark you made in Step 4. If you're using a thermometer, also record the temperature at these intervals.

Note: The time it takes to notice a change in temperature and water height may vary depending on the heat source you use and the size of your water bottle.

Side-by-side images of a notepad with data and a graph

8. Draw a conclusion

Write a description of what you observed in the straw. Graph your measurements on paper or using spreadsheet software.

What happened to the water level as heat energy was added? How does this relate to increasing global temperatures and sea level rise?