People liked that. It was nice to hear him admit that a thing could be better on Eden than on Earth, even if he was only joking. We smiled at each other, and some clapped.

‘Useless tree,’ Candy shouted, and a little cheer went up from the crowd.

‘What sound does it make?’ a man called out.

‘Our trees don’t make any sound,’ said Gaia, ‘except when the wind blows through them.’

How strange that seemed. In Eden there was no sound that was more constant in our lives than the sound of trees. They were pumping pumping pumping all round us right now, as we looked at the pictures and listened to the Earth people speak. Even out in Knee Tree Grounds, ten miles away from Mainground, that sound had always been there while we were awake and while we slept, like the heartbeat of the world. How weird it must be to live among silent trees.

‘How does it get its heat then?’ asked a woman.

‘Our trees aren’t hot,’ Deep said. ‘They don’t give out heat, like yours do. Or light either. Those green things at the top are called leaves, and they don’t give out light, they take it in. They take in the light of the sun, which they need in the same way that we need food.’

People nodded. We remembered that was part of the old stories, that the trees on Earth reached up their branches and drank in the light that came down from the star they called the sun. But I’d always imagined them with little mouths reaching upwards from the tip of every branch, open to the sky. In fact I’d carried a picture of that in my mind since I was a little kid: the trees with their little wide-open mouths that fed on light. In my head, those trees had always seemed kind of friendly, somehow, but this one just seemed strange.

‘That little girl was right,’ another man said. ‘No heat or light. Earth trees are useless.’

Several people cheered again.

Then the picture changed once more. There was more colour this time, and at the same time more darkness, which made it seem a little more like Eden.

‘Ah,’ said Gaia. ‘This is another picture from the beach. It’s what we call a sunset. It’s what happens when the sun goes down at the end of the day.’

The water was dark now, but where the sky should be, above World’s Edge, everything was bright and full of colour: pink, blue, green. It was the opposite of our Worldpool, where the sky is dark and the water shines. In middle of this strange sky, there was a small round circle of bright bright yellow. A long line of light stretched down from it across the water, broken into pieces by the waves.

‘This is the sun,’ Deep said.