What happens when you punch a tub of cornstarch-and-water — and when you press it slowly?
After you watchWhat happens when you punch a tub of cornstarch-and-water — and when you press it slowly?
The short answer
It's the same goo both times — only your speed changes what it does. A fast punch gives the cornstarch grains no time to move, so they jam shoulder to shoulder and the surface acts like a solid that your fist bounces off. A slow press lets the grains drift apart and the water flow into the gaps, so the same goo acts like a liquid your finger sinks through.
Try this next
- What if you used way more water? Imagine thinning the mix until the grains sit far apart — predict whether a fast punch still bounces, or whether it just splashes now.
- What if the fist never stopped? Picture stirring fast forever versus stopping dead — predict the exact moment the 'solid' turns back into a puddle.
The whole story
How it works
Cornstarch-and-water is packed full of tiny starch grains with a little water in the gaps between them. When you push slowly, the grains have time to shuffle out of the way and the water slips between them like slippery juice, so the mix flows like a liquid. When you punch fast, the grains get slammed together faster than the water can squeeze out from between them — they lock shoulder to shoulder into a jam with no room to move, and for that instant the goo resists like a solid. Stop pushing and the grains relax apart again, so it goes back to runny.
What people get wrong
People often think a substance has to be either a solid or a liquid, fixed for good — so the goo must 'really' be one and only seem like the other. Really it is neither in a fixed way: the very same mix is liquid when pushed gently and solid when hit hard. What flips it isn't temperature or a new ingredient, it's only how fast you push.
The catch
The hard, solid feeling only lasts a blink — the moment you stop pushing, the grains drift apart and the 'solid' melts back into goo, which is exactly why someone running across oobleck sinks the instant they stand still. And the trick only works when the grains are packed tight: add a lot more water and they sit too far apart to jam, so even a fast punch just splashes.
Questions kids ask
Is the goo a solid or a liquid?
Neither in a fixed way. The very same mix flows like a liquid when you push it slowly and locks up like a solid when you hit it fast — how fast you push is what decides.
Why can people run across a pool of it but sink if they stop?
Each fast footstep jams the grains into a solid for a blink, holding the runner up. The moment they stand still there's no fast push, so the grains drift apart and they sink into the now-runny goo.
Why does punching it not splash like water?
A fast punch slams the grains together faster than the water can squeeze out from between them. With no room to move, the grains jam shoulder to shoulder and stop the fist instead of letting it splash through.
How is quicksand different?
Quicksand is the opposite: struggling fast makes it looser and runnier, so thrashing pulls you down. Cornstarch goo goes harder when you push fast; quicksand goes softer.
Talk about it
- Ask them: it felt rock-hard for a split second, then went runny again — so where did the 'solid' go the instant the fist stopped?
- Ask: nothing was added or heated between the two pushes, so what was actually different about the fast one versus the slow one?
For grown-ups
This is a shear-thickening non-Newtonian fluid: a dense suspension of starch particles in water. Under slow shear the particles flow past one another with the fluid lubricating their contacts, giving a low effective viscosity. Under fast shear they are forced into transient force chains ('jamming') faster than the interstitial water can flow out of the contacts, so the effective viscosity spikes and the suspension resists like a solid; remove the stress and the chains relax. Quicksand is the opposite case — a shear-thinning mix that gets looser the harder you struggle.
Keep going
What else makes you wonder?
- If hitting it makes it hard, what's the fastest a custom mix could go solid — could you ever run across a whole pool of it?
- Quicksand does the opposite and gets looser the more you fight it — what tiny difference flips a goo from one behavior to the other?
- Ketchup, toothpaste and whipped cream all change how runny they are when you push them — which way does each one flip, and why?