Bill Sampson

Why?

So that's an oxygen atom and a hydrogen atom, which are bonded together.

And one's a bit more positive and one's a bit more negative?

And paper would be lumps of jelly on the floor as well.

So this bonding between an OH on one fibre and an OH on another fibre is crucial.

They're kind of sticking together.

The paper-making process of refining the fibres then squashing them together allows lots of sticky hydrogen bonds to form.

And those hydrogen bonds are partly why paper folds so perfectly.

So the only way to get rid of the crease would be to sort of pull it all apart into fibres again and put it back through the mill.

So that's the simple answer to listener Haruka's question.

Paper folding is irreversible because it permanently breaks some hydrogen bonds, as well as some of the fibres themselves.

But importantly, not all of them, which is why it folds like a hinge rather than breaking in two.

Now, not all paper folds equally.

For example, it's easier to put a sharp crease in printer paper than in, say, a paper napkin.

Why is that?

And what's the best paper for origami?

That's what we're looking at next.

This is CrowdScience from the BBC World Service, the show that explores your science questions.

This week, we're looking for answers to listener Haruka's question.

Why is paper so good for folding?