That's one of the theories in the Are Maiden's little book of wild ideas.

Most likely,

The fire making this smoke is somewhere around there, isn't it?

The end.

*From Perfect Memento in Strict Sense, pp. 122-124

They also say stuff like,

her ability's way too strong for a human,

or you can't really tell what she's thinking, don't they?

...Say.

Is she really human?

Haah...

You're blind as a bat, aren't you...

Of all the things to say.

Wha-!?