How to Make Oobleck: The Goo That's Solid and Liquid

Two ingredients, five minutes, and you get one of the strangest materials a child can hold. Punch it and it feels like a solid wall. Rest your fingers on it and they sink straight in. Squeeze a handful into a ball, open your hand, and the ball melts and drips away.

The name comes from Dr. Seuss's book Bartholomew and the Oobleck, in which a sticky green goo falls from the sky. We like to colour ours green too and call it zombie goo. It's one of the favourite experiments in our Pumpkins & Potions lab.

Materials

• 250 g (about 2 cups) cornstarch
• About 150 to 250 ml (2/3 to 1 cup) water
• Green food colouring (optional)
• A large bowl, a baking tray with edges, or a plastic tub
• Optional for spooky play: googly eyes, plastic skeleton hands, bones, spiders or bugs

How to make oobleck

1. Pour the cornstarch into the bowl.
2. Add a few drops of food colouring to the water, if using.
3. Pour in about two thirds of the water and mix with your hands. It will feel strange and stiff at first.
4. Keep adding water a little at a time until the mixture flows slowly when you tilt the bowl, but feels hard when you tap it quickly.
5. Too runny? Add a spoonful of cornstarch. Too crumbly and dry? Add a few drops of water.

The right mix is about 1.5 to 2 parts cornstarch to 1 part water. Cornstarch brands differ a little, so trust your hands rather than the exact numbers.

Things to try

• Punch it. Hit the surface quickly with your fist. Then rest your hand on it gently. What is different?
• Make a ball. Scoop up a handful and roll it quickly between your palms. Now stop and open your hand.
• Zombie hand test: can you slowly sink your hand to the bottom? Now try to pull it out fast.
• Rescue mission: hide plastic skeleton hands or spiders at the bottom and let children rescue them. Slow fingers work better than fast ones.
• Drop test: drop a small toy from different heights. Does it sink, bounce, or get stuck?
• Monster footprints: press toy feet or a fork into the goo quickly, then slowly. Which leaves a print?

What's happening? Non-Newtonian fluids

Most liquids, like water or juice, have a fixed thickness. Stir water gently or hard, and it stays just as runny. Scientists call these Newtonian fluids, after Isaac Newton, who described them.

Oobleck is a non-Newtonian fluid (A liquid whose thickness changes depending on how hard or fast you push, stir or squeeze it.). Its thickness changes depending on how you treat it.

Cornstarch doesn't dissolve in water. Instead, billions of tiny starch grains float in the water, each one surrounded by a thin layer of it.

• When you move slowly, the water has time to flow between the grains. It works like a lubricant, so the grains slide past each other and the mix pours like a liquid.
• When you hit or squeeze it fast, the grains are pushed together so quickly that the water can't get out of the way. They jam against each other, rub, and lock up, so for a moment the oobleck behaves like a solid.

As soon as you stop pushing, the water seeps back between the grains and the oobleck melts again. Scientists call this behaviour shear thickening (When a liquid gets thicker and harder the faster or harder you push it.).

Ketchup is the opposite

Ketchup is also non-Newtonian, but it does the opposite of oobleck: it gets runnier when you shake or squeeze it. That's called shear thinning, and it's why ketchup refuses to come out of the bottle until you give it a good shake. Toothpaste, paint and yoghurt behave the same way.

Try it: tip a little ketchup onto a plate and push it slowly with a spoon, then stir it fast. Now do the same with oobleck. One gets thinner, the other gets thicker.

Oobleck in the real world

Quicksand is a mixture of sand and water. Thrash around and it grips you; move slowly and water flows back between the grains so you can wriggle free. That's why the rule for quicksand is "move slowly"! Engineers are also studying shear-thickening liquids for flexible body armour and sports padding that stays soft until it's hit.

Make it spooky: zombie goo

• Colour it green, add a drop of black for a swampier look and call it zombie goo.
• Bury plastic skeleton hands so the fingers poke out of the surface.
• Make ghost goo with no colour at all: it's naturally white.
• Dancing oobleck (adult demo): pour a thin layer into a baking tray, cover a speaker with plastic wrap, place the tray on top and play a low bass tone. The oobleck wriggles and forms "fingers". Keep the oobleck well away from any uncovered electronics.

Cleaning up

• Scrape as much as possible into a bag or the bin.
• Let spills dry: dry oobleck turns back into powder that you can sweep or vacuum.
• Wash hand, tools and surfaces with warm soapy water. Rinse the sink with plenty of water after use, and never pour oobleck down the drain.
• Want to keep it? Cover the bowl and use it the next day. If it has dried out, add a little water. Throw it away if it smells sour.

Questions to ask

• Is oobleck a solid or a liquid? Why do you think so?
• Why does it feel hard when we punch it?
• Why does the ball melt when you stop rolling it?
• Can you think of other things that change when you push them fast or slowly?

Enjoyed the goo? Try our six slime recipes, from ghost slime to frog-spawn potion.

Frequently Asked Questions