This incredibly simple science experiment will keep your child entertained for hours and will take just a few minutes to make! It offers great fine motor skills as it can be a little difficult to mix, and is very intriguing to children because the goop has the consistency of pasta dough when you are handling it, but the minute you stop moving, it liquefies. If you start touching it again, it hardens, and again if you stop moving, it becomes a thick liquid and drips out of your hands – fascinating!

This is an activity that is inexpensive to make and thrilling for children. Plus, you can store it in a mason jar or any other reusable container, so you get several days of play out of it! This mixture is safe if children happen to put it in their mouths but, of course, you should avoid when possible.

How to Make It


  • A large spoon
  • ½ cup of corn starch
  • ½ cup tonic water
  • A black light or black light flashlight – I know that this is a little bit of an investment, but a black light is fascinating to children, offers hours of play, and can be used for many, many other activities I will be posting.

Let’s Get Started!

  • Your child can measure and put the cornstarch in a medium bowl – preferably a glass bowl so your child can see what is happening better.
  • Your child can slowly add some tonic water to the mixture, observing what happens, and stirring with a spoon (the larger the spoon, the easier it will be for your child to stir.) The mixture will be quite hard to stir so you may need to help. This magical goop gets hard when it is handled, then soft when left alone. It will look something like this:

  • When it is finally completely stirred, it will have the consistency of pizza dough. If not, adjust the proportions of cornstarch (to make it more solid) or tonic water (to make it more liquid)

Let’s Play!

  • You are now ready to play! Plug in the black light above the play area (best to play with it in a room that you can make darker by closing blinds and turning off the light) – a bathroom is ideal. Now prepare to be amazed by this goop!
  • Your child can hold it and play with it as if it was play-dough, but as soon as they stop moving their hands, they can watch as the goop magically turns into a liquid and goes back in the bowl!

Learning Opportunities

Children will learn S.T.E.M., especially science as they prepare the goop and math when measuring. They can learn about what solids, liquids and gas are (you can show them that the bubbles in the tonic water are a gas). They can learn about black lights and what makes things glow in the dark. I’ve prepared an easy explanation for you here. They will also be putting their fine motor skills to work, which are essential for writing.

CEFA tip: Remember to let your child do as much of the process as they are capable of. This means they pour, and they mix!

Extended Learning Opportunities

Now that your child has had the opportunity to work with a black light, find other things that glow in the dark:

  • Clothes
  • Toys
  • Teeth
  • What else can you find?

Watch some of the videos I suggested in my learning file to see how some animals are bioluminescent, and talk about those!

What else was your child interested in related to this game? Extend the learning by following their curiosity! If you need help with new games, simply leave a comment and I will try to send you ideas for you and your child.

Things to Keep in Mind

 Make sure you do this experiment on a surface that is easy to wipe.

Toys and Educational Material You Might Like

Click the images below to see where you can purchase these items.

For 0-3 year old children:

For 3-5 year old children:


Books You Might Like

For 2-5 year old children: