We have been engaging in lots of science experiments at my Montessori Pre-school recently. One that received a particularly positive reaction from the children (and the parents!) was 'Homemade Gak'. It was a pretty remarkable (and instantaneous) example of how a chemical reaction can change the state of molecules - it's also easy, fun and provides lots of sensory engagement! You can find lots of blog posts about it online - clearly it's popularity is pretty universal - but I thought I'd add our experience too as we enjoyed it so much!
White craft glue (we used Elmer's brand)
Borax (we tried initially with some other powdered soaps but only Borax worked for us!)
Food colouring and/or glitter (optional)
Squeeze the whole bottle of glue into a bowl (keep the lid, you'll need it in a moment).
Fill the empty bottle with water then replace the lid and shake. Open and pour the water in with the glue. Stir slowly. Add the food colour if you'd like to colour your 'Gak'.
Measure half a cup of warm water. Measure a teaspoon of Borax and stir it into the warm water.
Slowly poor the Borax/water mixture into the glue mixture while stirring. You'll start to feel it binding and transforming - as it coagulates you can use your hands to mix.
"Gak" - a sort of dry slime. It's stretchy and feels almost organic in its movements but it's also CLEAN and easy to control. It bounces when you roll it into a ball and it settles into a satisfyingly smooth shape when you leave it alone.
Why does it happen?
You are creating a "polymer".
When the glue or water is on its own it has long molecule strands that are separate and slide past each other, so it's in a liquid form. Combining the Borax causes those molecule strands to stick together, so it's in an "elastomer" solid form.