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While our bodies are at rest the large muscles are not in use.

It is these large muscles which pull on our bones and let us move as we want to.

Hard work with our large muscles uses up energy quickly so that we come to feel hungry and want to eat.

The more we know about the parts of our bodies and their work, the more new questions come to our minds,

questions such as:

How is it possible for all the different parts of us to grow from one cell?

How is it possible for all the millions of cells on our growing bodies to go on doing what they do without our being conscious of their working?

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Through about two billion years, scientists say, the development of plants and animals on our earth has been going on.

At first both were very small, much less than a pin point in size.

We might say that the first plants or animals were bodies living in one-roomed houses,too small for the eye to see.

All the housekeeping went on in one room.

Such cells take food in and let waste out.

They do not give birth to new cells but divide to become two daughter cells.

These three pictures show the division of a cell into two separate cells, each of which may in turn divide into two more.

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The earth itself is thought to be more than twice as old as life on it.

Our picture of what it was like two billion years ago is of a great ball about as hot on its surface as boiling water.

In time,as the earth's surface became less hot, the thick clouds round it,from which water went on raining down,got thinner.

Then more sunlight could get through to the surface of the earth.

Sunlight is necessary for life on earth,as air and water are.

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Life began, people think, in the salt sea.

Near the land, where the water were not deep, very small one-celled organisms came into being.

The cells in our bodies are of many different sorts but they all have water in them.

Our bodies are seventy per cent water.

This water has in it about the same amount of salt,

common table salt, as there was in the water of the sea in which the earliest organisms swam.

That water was a little less salt than sea water is today.

In billions of years the rivers have washed down more and more salt from the land into the sea.

The sea has become more salt.

The water in our bodies does not become more salt.

Even if we eat a great amount of salt in our food the body automatically controls how much it will keep in the blood stream.

And the same is true if we let very much salt out through the skin when we get over heated.

The we have to eat something that will give us new supplies of salt.

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The first living things, though they were too small to be seen, were eating,growing organisms which divide to become others like them.

Under a microscope, a cell looks like a bit of clear jelly with a thin wall round it.

Very small openings in the walls of the cell let food in and waste out.

In every cell there is a part like a little ball.

This is a the nucleus, Which organizes the work of the cell.

Those cells were discovered two and a half centuries ago,it is only in the last hundred years that knowledge of the work of the nucleus has developed.

It is this nucleus which keeps the cell working at what has to be done.

It is to the cell what London is to Britain,New Delhi to India, Beijing to China.

It is the seat of government.

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All living things are developments from these one-celled organisms.

They have developed step by step through change upon change from these first, simple,living things.

Our bodies and bodies of the plants and animals we see about us are highly complex in their structure.

They have in them millions of millions of cells, some doing one sort of work, some another.

Every different cell has its own sort of work to do.

What do the words simple and complex mean?

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The greater the number of parts a thing has, the more complex it is.

The smaller the number of parts a thing has, the simpler it is.

Which of these is the more complex?

The more different the parts of a thing are from one another, the more complex it is.

The less different the parts of a thing are from one another, the less complex it is.

Which of these is more complex?

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The more the parts of a thing need one another, the more complex it is.

Take one little wheel out of a watch,for example, and see if the other parts can do their work without it.

Which is more complex- a stone or a plant?

The leaves, the flower,stem and roots of the plant work together to keep it alive.

But each does a different sort of work and each is dependent upon all the others.

The work of each part is different.

The parts of a stone do not work in this way for one another.

A stone is a much simpler thing than a plant.

Any living thing,however small or simple it may seem, is far more complex than anything which has no life.

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People are the most complex of all organisms, far more complex than any plant.

We would not go on living, but would die, if the millions of millions of cells in our bodies did not work together in their different ways, each doing its own sort of work.

The cells are dependent upon one another and we are dependent upon our cells.

The picture below shows what the word dependent can mean.

The girl in the middle is dependent on both men to keep her from falling.

The man who has been holding her feet is letting go as the other man takes hold of her hands.

She has been dependent on first man.

She will be dependent on the second.

Their act depends on timing.

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There are about five million red cells in one cubic millimeter of our blood.

One millimeter is a thousandth part of a meter and a tenth of a centimeter.

One cubic centimeter-how much space does that take up?

Here is a picture of a cube which is one centimeter long,one centimeter wide and one centimeter high.

It takes up one cubic centimeter of space.

A centimeter is a little less than half an inch.