[kruuuder]

If you have read the story and, like me, are still wondering which part of the story is the quote at the top of the post:

"It's Odin's Disc. It has only one side. Nothing else on Earth has only one side."

[tines]

A mobius strip does!

[bibanez]

A mobius strip made out of paper has 2 sides, the usual one and the edge.

[tines]

You can make it out of some other material that you can smooth, i.e. so that it doesn’t have edges.

[tines]

Come to think of it, doesn't a sphere have only one side?

[WJW]

How about things like Klein bottles that have no edges? (Although I guess that unlike a Mobius strip it's not possible to make a real one here on Earth so the quote from OP still holds)