What is green slime?
The purpose of a scientific investigation is to make a discovery. From discoveries, scientists can develop ideas about everything in the universe. From these ideas they can provide us with explanations of how everything works - from the growth of a flower to the explosion of a star. Sometimes scientists can come up with a simple idea, which can explain how a lot of things work. The idea that all matter is made from particles is a good example. This idea was developed long before scientists could even see the particles. When microscopes that were powerful enough to see the particles were invented, scientists everywhere were excited. The microscopes proved what they had believed for a long time.

One of the problems with the idea of things being made from particles is that people think of the particles as grains of sand or specks of dust. The word "particle" can be used in this way but in science it means something much, much smaller. If you can remember this then, when you look around you, you can explain what you see in terms of particles.

Imagine you are having an ice lolly. The ice is made from particles that are locked firmly together. They give the ice lolly its shape. Someone telephones you and you put the ice lolly down on a sunny windowsill. As you talk with your friend the particles of ice receive energy from the Sun's heat and shake about. They slide past each other and form a liquid that flows along the windowsill. As you talk, the liquid becomes so hot that some of its particles move fast enough to escape. Evaporation takes place and water vapour forms. Later the water vapour strikes the cool surface of a drinks can you have taken from the fridge. The water particles lose energy, condensation takes place and droplets of water form on the can. Two hundred years ago people could not explain these changes with the confidence that we can do today.

However, one of the great joys of science is that just when everyone thinks they know how something works they discover something that does not fit in with their ideas. The key to explaining how solids turn into liquids is heat. Substances take it in to change from a solid to a liquid and lose it when they change from a liquid to a solid. When you make a mixture of cornflour and water you can change a solid to a liquid and back again without using heating and cooling. How can this be?

When scientists investigated this discovery they found that you have to think about the common use of the word particle. Cornflour is made up of particles that you can see. They form a white powder. When you mix cornflour with water the particles in the powder spread out in the liquid. They do not dissolve. They are so small that they do not settle out like sand grains but hang in the liquid as if they were suspended there. In fact, a mixture of water and small particles like this is called a suspension. In a normal mixture you just get a white runny liquid but when you mix a certain amount of cornflour with a certain amount of water something else can happen. If you pour this mixture onto your hand it flows across your palm and through your fingers. If, however, you draw in your fingers quickly, as if you were trying to grab something, the liquid immediately turns into a solid ball. If you try and gently pick up the ball, it turns into a liquid and flows away. All these changes are brought about without a change in heat. The key to this change is the sudden jolt that you give the liquid. This makes all the powder particles lock together and form a solid. As soon as they recover from the shock of the jolt they lose their grip of each other and the mixture becomes a liquid again. If you add green colour to the mixture it looks like slime and you can entertain your friends with it.

While the green slime made by cornflour and water is a fascinating material which will give you plenty of fun it also helps you realise that there are always surprising discoveries to make and just when scientists think they know it all they find they don't.

What is real slime?
It is made by living things. If you have been to the seaside and looked in rock pools you may have slipped on seaweed. It has a coating of slime. Seaweed needs a certain amount of water in its body to stay alive. Its slimy coat helps it to keep water in its body while the tide is out. A frog has a slimy skin. This helps it to keep water in its body when it is out of its pond. The slime also makes it difficult to catch. It makes the body as slippy as a bar of soap.

Are snakes slimy?
Many people think so but they have a dry skin covered with scales. The scales fit together so closely that they make the skin very smooth and sometimes slippery but they do not have slimy bodies.

Slugs look slimy. Do they have slime?
Yes, they do. The slime helps them to keep water in their bodies. It also helps them to walk. If you look behind a slug or a snail you will see that it has left a trail of slime. It makes slime on the underside of its body then moves over it.

Why do people find slime revolting?
People make a kind of slime called mucus. It helps to keep the air passages in our nose and throat clean and healthy. When we are ill we make a great deal of mucus to kill off the germs that affect our airways.

Why is the cornflour and water mixture called slime?
Because it behaves like slime. It flows over your fingers slowly. It does not have a purpose, like the slime made by living things, but shows this unusual feature of turning into a solid when you squash it suddenly. Natural slimes do not have this property.

Can you make any other slimes?
Yes, there are quite a few. A simple one to make is called green glue slime. This is made by mixing white PVA glue with a substance called borax then adding green food colouring. If you mix the ingredients in a certain way you get a smooth rubbery substance which flows slowly in a slimy way. If you grip it, it turns into a solid and if you throw it on the floor it bounces then starts to spread out slowly to form a slimy pancake. Some people are allergic to borax so when this slime is made it is best to wear rubber gloves. When you have finished making slimes put them in the waste bin. Do not put them down the sink because they might block the drains.