Walking rainbows or a lesson in capillary action

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Walking rainbows or a lesson in capillary action

You've probably guessed that bright colours make me happy and that I'm drawn to any experiment that is bright and colourful -- that's why today's experiment was one that I had been wanting to do for ages.

I’m well aware that there are plenty of examples of this experiment all over the internet. Even still, I wanted to do this experiment with Lily and Ethan and I took the opportunity to capture these lovely photos.

There is another reason that I wanted to put together my own post about the walking rainbow. A lot of the posts that I have come across present this as a pre-school experiment with very simple explanations of the science. I’m a huge advocate of doing experiments with little ones and talking with them in a language that they understand but I think that sometimes we dismiss experiments as being “simple” or “basic” when in fact they have the potential to teach science at a much higher level.

So, for this experiment, I have given three levels of scientific explanation; Pre-school to early school years, years 2 to 3 and years 4 to 6. This is the first time that I have extended explanations out in this way and I would really value your feedback as to whether it is useful!

First things first though, this is what you will need:

  1. 6 jars or glasses. I have found that larger baby food jars are a great size for quite a few of our experiments but smaller jam jars or even drinking glasses work perfectly well.
  2. Liquid food colour in red, yellow and blue. I like the Queen brand in the individual bottles because they give a bright result but I’m sure most brands will work fine. Also it doesn’t have to be primary colours but the discussion about colour mixing is easier with those.
  3. Kitchen paper towel. Look for something with good absorbency. Home brands are often not that great.
  4. Scissors. They need to be sharp enough to cut paper towel so kids scissors may not do the job.
  5. Water. Room temperature water from the tap is fine.

Coloured water in jars with paper towel connecting them 


  1. Fill three of the jars with water, close to the rim.
  2. Add a couple of drops of food colour to each (avoid making the water very dark with colour ¼ of a ml should be plenty.
  3. Arrange the 6 jars in a circle and alternate the jars with water with empty jars.
  4. Take 3 squares of paper towel and cut them in half. Fold each of the 6 pieces in thirds to make a strip.
  5. Create a paper towel bridge between each jar by placing one end of a strip in a jar with colour and the other into an empty jar. Do this to make a connection between each of the jars as in the photo below.
  6. Wait and observe. 


Although you will see the coloured water begin to soak into the towel immediately, there is still plenty of time to get the kids to make a hypothesis about what will happen. Talk about each of the strips of paper and what colours you think they will become and also talk about what you think will happen to the empty jars.

 Picture of coloured water that has travelled through paper towels to another jar

The science

Pre-school to early school years

Explain that paper towel absorbs (sucks up) liquid. Once the paper towel has absorbed all of the liquid that it can hold, the extra liquid will flow or drip off the towel and start to fill up the empty jar. Because there are two towels carrying different coloured water into the empty jar, those colours will mix and a new colour is made.

To extend this experience, you might want to give the children the opportunity to do some water play with different colours of water and talk about the new colours that they have made. You could also use paints for the same purpose if the weather is not appropriate for water play.

Years 2 to 3

Start with a discussion about gravity and ask the child or children if water normally travels up out of a glass or jar if you put something like a straw in it. Most kids will answer “no”. You could talk about the reason for that, which is that in normal circumstances gravity is keeping the water in place in the absence of any forces acting against it.

Now you can introduce the concept of capillary action. Capillary action was first observed and recorded by Leonardo Da Vinci. Capillary action occurs where a liquid such as water will travel upwards in very narrow tubes (the narrower the more effective) and will defy gravity!

You can also talk about saturation of the paper towel and the fact that when it is full of liquid, the excess liquid will move into the available space, which is the empty or jar that has less liquid.

You could extend your study of capillary action by placing a white flower with a longish stem into a glass of coloured water. After a day or 2 the petals of the flower will take on the colour of the water!

Years 4 to 6

For years 4 to 6, you can start with the same discussion as for years 2 and 3.

Then move on to discuss the fact that the water kept moving from the jar that had more water to the jar that had the least water until they reached an even level. In this case, the paper towel is acting as a siphon (most often a tube), the capillary action creates the force needed to get the water from one jar moving through the siphon. Once the action has begun water continues to move from the jar with more water to the jar with less water until the water levels are even. The reason that the liquids reach an even level in the jars is that the air above the water exerts pressure that pushes the water through at the higher level through the siphon until the levels on both sides of the siphon are the same.

To extend this experiment, try working with ice cold and warm water and test whether the rates of water movement are the same or different.

If you and the kids enjoyed this, take a look at our coloured flowers experiment!

And if your kids can't get enough of science experiments, then check out our Chemistry-in-Action kit! We promise you won't be disappointed.

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