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There is not enough petrol in the world for everybody now, and each year there is less,
so what are we going to do when it finishes?
Perhaps we will go back to horses and carriages and bicycles.
In the Second World War, some people did not use petrol in their cars.
They made gas from wood and plants instead, and then they put it in big bags on top of their cars.
The cars did not go fast, but it was better than nothing.
But we cannot cut down all our trees to make gas; we need them for other things too.
Besides gas, we can also use electricity for our cars, but first we must make the electricity!
Some countries have coal, and they make electricity with that, but we'll not always have coal.
Other countries have big, strong rivers, and these turn turbines and make electricity more easily and cheaply.
We are also able to get power from the tides.
We put turbines in the mouth of a river.
Then, when the tide comes up, it turns the turbines,
and when it runs back towards the sea, it turns them again.
And we know that the waves of the sea can also turn turbines when they go up and down.
Which of all these things will make our electricity in the year 2000?
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