A very strange liquid-solid!

• 1 bowl for mixing
• 1 cup of water
• 1 fork for stirring
• 1/2 a cup of cornflour
• 3-6 drops of food colouring

Scrape the slime out of the bowl. Roll the slime into a ball and throw it onto the kitchen table.

Did you notice that it 'melts' into a runny (viscous) blob when it sits on the table but it's hard and crumbly (brittle) when you squeeze it or squash it? When the slime is a blob on the table poke it hard with your finger. Can you get your finger through the slime? Now gently press the slime with your finger. See how it slips easily through the slime

What you have just made is a special sort of substance called a 'stir-thickening' liquid. When you stir the slime you are adding moving energy (kinetic energy) to it. This kinetic energy makes it change. It changes by becoming thicker and harder. When you stop stirring it changes back and becomes runny again.

The chemical name for when things get thicker when you stir them (stir-thickening behaviour) is 'dilatancy'. The opposite of dilatancy is called 'thixotropy' or 'stir thinning'. There are not many stir-thickening substances around your home but there are alot of stir-thinning substances as you can see below.

Your home is full of stir thinning liquids.

Tomato ketchup, honey, emulsion paint, toothpaste and mustard are all stir-thinning liquids. If you want to get ketchup out of its bottle you shake it up to stir it and this makes it thinner and runnier.

If a big pot of paint has been left a while it will become thick and you cannot get it out of the pot. If you want to get it out of the pot then you have to stir it up and this makes it runny again.

The ink in ball point pens is also stir-thinning. If you turn a biro up-side-down the ink will not flow out...so how come you can write with it?

Well, at the tip of the biro is a small steel ball which churns up the ink as you write (that's why it's called a 'ball point' pen). Since the ink is stir-thinning it becomes runny and flows out of the pen and onto your paper, but only when you roll the ball in the pen by writing with it...clever eh?