Hands-on science: Sticky Rice
Sticky Rice
Grains versus gravity
A knife normally goes through uncooked rice like, um, a knife through butter. But persevere and you can get enough friction for a surprising lift.
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Step by step
Yan's video guide
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Dr Yan invites some friends to see who can get Sticky Rice to work first
Checklist
| Difficulty: low | Easy to do once you find the knack |
| Time/effort: be patient | Can take a few minutes |
| Hazard level: low | Take note of the safety advice and options |
SAFETY: Obviously, take care with knives and make sure glass jars don't fall and smash.
Younger children should use one of the suggested alternatives to a knife and also a plastic container.
You need
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Either: a clean plastic container such as a 300-500ml drink bottle
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or: a clean glass jar (eg jam jar)
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Enough long grain rice to fill the container or jar right up
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A blunt thin-bladed knife, eg a butter knife
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Alternative: a pen, pencil or ruler
What you do
Pour rice into the container almost to the top.
Try to push the knife into the same place each time
Hold the container firmly with one hand. With the other, push the knifeblade (or pencil etc) straight down into the rice.
Pull the knife back up and out but don't quite remove it from the rice. Then push it in again. Keep lifting and re-inserting the knife at a slow steady rate.
If the level of rice in the jar starts to drop, add more rice to fill it back up.
What should happen
The rice starts to compact as the grains fit together more tightly and air gaps shrink.
It could take only a few strokes or it could take dozens - but you should feel that it becomes hard work to pull the knife out.
Eventually, with a bit of luck, instead of the knife sliding out, it gets jammed in so firmly that it will happily support the weight of the rice and the container.
When you try to remove the knife blade, the jar lifts up instead.
As soon as the rice has gone 'sticky' once it will probably be quite easy to get it to set solid again. So if you're challenging a friend, give the container a really good shake (with a lid on) to loosen the rice up first.
If it doesn't work for you
There is definitely a knack to this and it's hard to explain. Some people get sticky rice quickly; others take ages.
Any long grain rice should work but you could experiment with different types. Some say that basmati rice works best.
The solid sides of a glass jar make the trick a little easier but plastic bottles do work and are safer for younger children.
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Avoid very soft plastic such as gets used to make milk bottles.
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A bottle or jar that narrows towards its neck makes things easier.
Don't stab randomly and don't stir the rice up. Try to keep the knife going along the same line each time. Gently shaking the jar occasionally can help the rice settle into place.
What's going on?
Increasing the friction
The flat shiny sides of a knife slide quite well over and through things but there is still friction there.
Similarly, rice pours and can normally be pushed around easily because there are large air gaps and the grains themselves touch only lightly over small areas.
Every time you plunge the knife into the rice you are compressing the grains and making them pack more tightly together.
The air gaps decrease in size and the rice grains rub against each other more. They can't move as freely, and start to arrange in a pattern that doesn't change. The rice you could previously pour like a liquid becomes solid.
With more grains of rice pressing on the knife blade, and each one of those more tightly packed in, the friction between the knife and the rice increases.
If the friction force between the rice and knife equals the combined weight force of the rice and container, then the balance of forces means the knife is held in place.
For the jar to lift as well, friction between its inside walls and the rice must also increase. That shows you the rice has moved and compacted even away from where you disturbed it with the knife.
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