A liquid that panics
Why Can You Run on Cornflour and Water?
Hit it hard and the grains jam together like a solid; press gently and they slide like a liquid.
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Step 1 of 5
It is a crowd of grains, not a smooth liquid.
Cornflour is millions of tiny particles suspended in water. The water fills the gaps between them and lets them slide past each other.
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Step 2 of 5
Slow pressure gives the water time to move.
Push gently and water flows around the grains, so they slip past one another easily. That is exactly how a liquid behaves.
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Step 3 of 5
A fast hit does not.
The grains are shoved together faster than water can get out of the way. They lock against each other and the mixture behaves as a solid.
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Step 4 of 5
The jam undoes itself immediately.
As soon as the force stops, water seeps back between the grains and the mixture flows again. It cannot hold you up if you stand still.
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Step 5 of 5
Plenty of real materials work this way.
Quicksand, wet sand at the shoreline and some body armour all change stiffness with how fast they are pushed. Speed is a property, not just a number.
Ketchup does the reverse: it thins out when shaken and thickens when left alone, which is why the bottle needs a shove.
The short version
Cornflour mixture jams solid when grains are shoved together faster than water can escape, and flows again the moment the push stops.
Try it yourself
Mix two parts cornflour to one part water. Punch the surface, then rest a finger on it and let it sink.
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