Homemade Slime Science: What Is Really Happening?

Slime is one of the few things children will happily spend an hour making, and it is real science. The stretch, the slow drip and the satisfying squish all come from the way very long molecules behave. You do not need a recipe tin full of chemicals to show that.

A note before you start. Slime is not food and should never be tasted. An adult should do the measuring and stay close, especially with younger children. Wash hands after play, keep slime away from hair, carpet and pets, and stop if anyone’s skin gets red or sore. If you use any product with a warning label, follow it and keep it away from young children.

The idea in plain words

Imagine a bowl of cooked spaghetti. Each strand is long and slippery, and the strands slide past each other when you stir. Now imagine the strands gently hooked together here and there. The whole bowl would hold together and stretch instead of pouring.

Slime works like that. The “strands” are polymers, which are very long molecules made of small repeating pieces. Glue contains polymers that slide around in water. A “linker” ingredient hooks them together loosely at a few points. You get something that is neither fully liquid nor fully solid.

That is why slime does two things:

Pull it slowly and it stretches and flows.
Pull it fast and it snaps, because the strands do not have time to slide.

Ideas that avoid borax

Many popular recipes use borax, which should not be handled by young children. There are gentler experiences that show the same ideas.

Cornflour and water. Mix cornflour (cornstarch) with water, a little at a time, until it flows slowly when tipped but feels firm when you press it. It is a different material from slime, but it shows the same “sometimes liquid, sometimes solid” behaviour. Our blog post on oobleck has the method.

Cooked flour paste. An adult cooks a spoonful of plain flour in water on the stove until it thickens, then lets it cool completely before children touch it. The starch molecules are long chains too, and the thick paste is a close cousin of slime. Hot liquids need an adult throughout.

Ready-made slime you already own. If a child has slime already, you can run all the tests below with it.

Five things to test

Use the same piece of slime and change one thing at a time.

Slow pull and fast pull. Which one stretches and which one snaps?
Stretch race. Pull to the longest length you can before it breaks and measure in centimetres. Do it three times and look for a pattern.
Drip test. Put a ball of slime on the edge of a plate, tilt it a little and time how long it takes to flow 10 cm (4 in). Does temperature matter if you try it after a few minutes in a cool room?
Bounce or flop. Roll it into a ball and drop it from 30 cm (1 ft). Then press it flat and let it sit. What changes?
Shape memory. Press a coin into it and leave it for ten minutes. Does it keep the shape?

Write down each prediction first. “I think it will stretch further when it is slow” is a testable idea, and testing it matters more than being right.

Words worth teaching

Polymer: a very long molecule made from small repeating parts.
Viscosity: how thick and slow-flowing a liquid is. Honey has high viscosity. Water has low viscosity.
Elastic: springs back after being stretched.

Children as young as five can use “stretchy” and “runny” correctly, and older children enjoy “viscosity” because it sounds technical.

Clean-up

Throw slime in the bin rather than down the sink, as it can block pipes. Wash tables and hands with soap and water.

Where to go next

The lesson Is oobleck a solid or a liquid? takes these ideas further with a proper investigation. Our kitchen science safety rules cover what to check before any mixing, and physical versus chemical change helps you decide what is happening when ingredients combine.