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You can put a paper cube this size together for yourself, or cut one out of soap or cheese.

To make a paper cube, take a pencil and make six one centimeter squares like this:

Now take your scissors and cut round the outer lines.

Do not cut the broken lines but make folds there.

Put the edges together to make a hollow paper cube,as in the picture.

Your cube will be a cubic centimeter in size.

A cube has six sides; its sides are equal squares.

The surface of a one-centimeter cube is six square centimeters.

In one cubic centimeter of our blood there are about five billion red cells.

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Living things are made up of cells, and cells do not grow to be more than twice the size they were at first.

But trees can grow to be many thousands of times the size they were as seeds.

The tallest tree known is 364 feet high.

One great tree in California is as much as 115 feet round its trunk at the thickest point.

How does such growth take place if cells do not grow to be more than twice as large as they were at first?

It all take place through division.

The cells which make up the organism are able to divide into two and this division goes on and on.

When you were born you had as many as 200 billion cells in your body all coming,in nine months, from the division of one cell 1?175th of an inch across.

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All organisms which you see in the world about you have grown by cell division.

You and I have become what we are through billions of division in the cells whose outcome is our cell structure at this minute.

In cell division the two halves of the cell (as you see in picture four) become ready to separate.

In picture five you see that they have separated.

The cell in picture one, by division, has become two separate cells.

The daughter cells do in every way the same as their mother cell did.

One of the greatest questions scientists are working on is:

What keeps the daughter cells doing what they have to do?

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Even the simplest living organism is far more complex than any machines that people have made.

Organisms are built up of parts which are themselves more complex than any machine.

And these parts in turn are the most complex things the science of chemistry knows.

It is because they are so complex that they can work together in an organism in so many different ways.

Chemistry is about different ways which different materials are built up.

For example:

Water is made up of oxygen and hydrogen united in a way which may be represented like this.

Carbon dioxide is make up of carbon and oxygen in a way which may be represented like this.

Chemists make use of the formula H2O for water in their writing.

They use formulas to represent the structure of all material things.

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The most important of the materials in our bodies are the proteins.

Among them are materials with the most complex structures known to the chemist.

We can get some idea of how complex proteins are by comparing the formula for one of them with the formula for water or for carbon dioxide.

Compare this formula for a protein form milk:

with the simple formulas for water:

or carbon dioxide:

The letter N this formula represents nitrogen and the letter S represents sulphur.

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There may be as many as 100,000 different sorts of proteins in the human body, at work in as many different ways.

It is protein in our blood which takes up oxygen from the air we breathe into our lungs and carries it to the cells which need it.

It is another protein which makes our bones strong, and another which makes our muscles able to pull on and move our bones.

It is other proteins in our hair and skin and nails which make them what they are.

Others again do very important work in the digestion of food, in controlling other changes in the body, in keeping it healthy and in making children become so surprisingly like their parents.

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How do these very complex materials come into being?

The answer is, through the work of plants.

Plants need sunlight.

This is common knowledge within the experience of most of us, but you can show how true it is if you make this little experiment.

Grow two bean seeds over glasses of water, starting them in a dark cupboard.

After a few days take one glass out into the daylight and keep the other in the dark.

Compare the two from time to time.

You will find that it is the plant in the light which grows a strong stem and green leaves.

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In 1772 the Englishman Joseph Priestley, the discoverer of oxygen, made a most important experiment.

He knew from experience that air is necessary to plants and animals.

He knew that if you put a live mouse,for example,under a glass so that no fresh air can come to it,the mouse in a short time will die.

It will have taken all the oxygen out of the air and without oxygen it cannot go on living.

Priestley put a green plant under a large glass cover standing in water so that no fresh air could get in.

He thought that in a little while he would find the plant dead like the mouse.

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But no.

Here is what he said:

When it had gone on growing there for some months, I found that the air would neither put out a candle nor was it bad for a mouse which I put into it.

In other words, the green plant had not used up all the oxygen in the air.

The next step was taken when a Dutch scientist found that Priestley's experiment would not work without light.

In the light a plant gives out oxygen and builds up sugars and other complex materials in itself.

In the dark it gives out carbon dioxide and water.

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The materials which makes plants green is necessary if they are to use energy from light for healthy growth.

Unlike animals, plants can take what they need straight from the air and earth and water in which they live.

Through their green leaves and stems they separate H from H2O and unite it with CO2 to make carbohydrates.

Structure of part of a carbohydrate, a sugar.

This power of plants is named photosynthesis.

People and animals depend upon photosynthesis in plants to supply their food and energy.

We and the animals either eat plants or eat plant eating animals or both.