Highlighters are so bright that you’d think their ink lasts forever. Make it disappear with a simple citrus fruit!

You will need

  • Yellow highlighter
  • White piece of paper
  • Cutting board
  • Knife
  • Lime (or lemon)
  • Cotton tips


You’ll need to use a sharp knife in this activity. Ask an adult to help.

What to do

  1. Find a flat surface, and then grab your yellow highlighter and a white piece of paper.

  2. Draw on your paper with the highlighter. You can make it artsy!

    swirly marks on a page drawn with a yellow fluorescent  highlighter pen.
  3. Get a cutting board and a lime.

  4. Ask an adult to cut the lime in half with a knife.

    a lemon cut in half.
  5. Gently push the cotton tips into the lime so that the tip soaks in the juices.

    Pressing the end of a cotton tip into the flesh of a lemon slice.
  6. Use the wet cotton tip to swab over the highlighter mark. What happens?

    Gif or a highlighter pen mark being removed by wiping it with a cotton tip soaked in lemon juice.
  7. Get creative! Make some highlighter artwork.

  8. Happy with your awesome piece of highlighter art? Now make it glow in the dark!

What’s happening?

Most yellow highlighters get their colour from a dye called pyranine. This special chemical is very good at reflecting yellow light. This means we see the colour yellow wherever the highlighter pen has gone.

You might also notice that the yellow ink seems really bright, almost glowing. This happens because pyranine is fluorescent. Fluorescent chemicals absorb light in some colours and emit them in different ones. In pyranine’s case, it absorbs blue light, and also ultraviolet light and bounces it back as yellow. You can test this out with our Glow in the dark activity!

Pyranine’s ability to absorb light and bounce it back comes from its chemical shape. But this shape is quite fragile. Here’s where your citrus fruit comes in! Citrus fruit like lemons and limes contain high amounts of citric acid. When pyranine mixes with acids, it changes shape so that it no longer bounces back yellow light. So, wherever your acid-filled cotton tip touches, the pyranine goes colourless.

Turns out this sensitivity to acid makes pyranine quite useful for chemists and biologists! They use pyranine to measure acidity, including in tiny spaces like the insides of cells. Pretty amazing for highlighter ink! To test out (and eat) another acid-sensitive dye, check out our Colour-changing noodle activity

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

By submitting this form, you give CSIRO permission to publish your comments on our websites. Please make sure the comments are your own. For more information please see our terms and conditions.

Why choose the Double Helix magazine for your students?

Perfect for ages 8 – 14

Developed by experienced editors

Engaging and motivating

*84% of readers are more interested in science

Engaging students voice