What you’ll need

  • 3 jars
  • teaspoon
  • blue and yellow watercolor paint
  • paintbrush
  • water
  • paper towels

How to do it

1. Line up the three jars in a row.

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2. Fill the first and third jar about three-quarters full with water.

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3. Wet the brush, dip it lightly in the blue paint, then make the water in the first jar turn blue. Next, color the water in the third jar yellow the same way.

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4. Fold a paper towel three times lengthways to create a narrow strip.

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5. Bend the strip in the middle and place one end in the middle jar and the other in the first jar, so that the strip touches the water in both jars. If the strip is too long and sticks up in the middle, make the strip a bit shorter.

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6. Take a second paper towel and repeat as above so it connects the middle and third jars.

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7. Allow the jars to stand and return frequently to observe them. What happens? After 15 minutes you should already be able to observe something.

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Well spotted!

As soon as you put the paper into the jars, the colored water starts to flow upwards. After a few minutes you see how the water starts dripping into the empty neighboring jar. After approx. 15 minutes you can already identify the colors in the neighboring jar. As soon as the water level is the same in all three jars (after approx. 120 minutes), it stops flowing. If you use the teaspoon to mix both colors together, the water in the jar turns completely green.

What's the secret?

Water and other liquids are able to flow uphill in narrow tubes or gaps, against gravity. This phenomenon is called "capillarity".  In the case of trees, capillarity is one of the things that ensures that water can climb up from the roots to the leaves.

The fibers in paper towels contain narrow cavities. Water moves along these hollow spaces – even uphill – when the paper towel is standing upright. As soon as the water arrives at the highest point – where the household paper is bent – capillarity no longer matters, because the paper on the other side is pointing downwards anyway. The water is moved by gravity and flows into the other jar.

As soon as the water in all the jars reaches the same level, the water stops flowing. This is the principle of communicating vessels. When two or more vessels are connected to each other (e.g., by a hose), the water in them always reaches the same level, no matter what the shape of the vessels or how much water they contain.

Blue, red and yellow are primary colors: they cannot be created from other colors. If you mix two of these primary colors together, you create a new color. If you mix blue and red, you get purple; mix red and yellow together and you get orange. Or mix blue and yellow to get green.