Arthur C Clarke - The Wall of Darkness - Mobius strip

“Again Grayle brought the two ends of the strip together, but now he had given it a half twist so that the band was kinked. He held it out to Brayldon.
’Run your finger around it now,’ he said quietly.
Brayldon did not do so, he could see the old man’s meaning.
’I understand, he said. ‘You no longer have two separate surfaces. It now forms a single continuous sheet – a one sided surface – something that at first seems utterly impossible.
’Yes,’ replied Grayle very softly. ‘I thought you would understand. A one sided surface. Perhaps you realise why this symbol of the twisted loop is so common in the ancient religions, though its meaning has been completely lost. Of course, it is no more than a crude and simple analogy – an example in two dimensions of what must really occur in three. But it is as near as our minds can ever get to the truth.’
There was a long, brooding silence. Then Grayle sighed deeply and turned to Brayldon as if he could still see his face.
’Why did you come back before Shervane?’ he asked, though he knew the answer well enough.
’We had to do it,’ said Brayldon sadly, ‘but I did not wish to see my work destroyed.’
Grayle nodded in sympathy.
’I understand,’ he said.