Dust

"Dust", or more specifically, magic dust, refers collectively to cosmic particles originating in the Wood between the Worlds, permeating its air and giving most sentient beings who breathed it in a revitalising or refreshing sensation (though after a while, its effects tended to overwhelm the beings' bodily functions). The particles may exist in any universe though they retain a powerfully inherent affinity that attracts them always toward the Wood between the Worlds. It has been proven that proper manipulation of dust particles can allow individuals to travel among different worlds and their distinct universes.

History
It is believed the substance was discovered first by the Atlanteans or the fairy people (there is some evidence that these two groups were in fact one and the same) who employed dust for spacetime travel (in particular, the Atlanteans). The first human documented to have experimented with dust (though probably not the actual first one to do so) was Andrew Ketterley in Earth-year 1900. The dust he encountered he had inherited from his godmother, Mrs. Lefay which had probably been passed down from distant fairy ancestors. Although Lefay ordered Andrew to destroy the box which held the dust, he kept it after she died and began to try to study dust and its properties. Even before opening the box or knowing what was inside it, he described that he felt a tingling sensation of magic just from touching the box. This feeling was probably emitted from the dust itself and when Andrew did look inside, although it merely looked like sand, he knew immediately it was some higher force that he should avoid touching directly. Incorporating the dust particles, he eventually fashioned two types of magic rings—a yellow variety and a green variety—that he hoped would allow the wearer to respectively go to or return from the "otherworld". Testing the yellow rings on guinea pigs and an unknowing girl, Polly Plummer, Andrew learned that they actually brought the wearer to the Wood between the Worlds (a sort of "in-between" area of the universes) when his nephew, Digory Kirke, returned with Polly from the Wood using a green ring and told him so. Andrew Ketterley himself later also traveled using the magic rings, although he did so involuntarily. Other known travelers who have used these dust-based rings include Jadis of Charn and Frank the cabby and Strawberry the horse of Earth.

Affiliation with living beings
There is some possibility that dust comprises the fundamental particle responsble for sentient life. As evidence, one must understand that dust, in addition to its affinity for the Wood between the Worlds, also demonstrated an affinity for sentient beings like humans. For example, if a human were merely touching another human who was wearing one of the magic rings, he or she could also travel to other worlds. In fact, an entire chain of humans could travel together with only a single ring among them. It is believed that the dust of the ring is what linked them together, allowing them to move as a single force.

Also, the Wood between the World could be perceived as a sort of base of operations for all worlds with life on them, using pools of water as different portals into different worlds. Interstingly, when a world (like the world of Charn) lost all living organisms, it dried up, suggesting at the very least, dust's affinity for living creatures and its possibility as the fundamental "life force" that unites all universes.