And this was just the start.

Drea’s eyes were bright. “Jump this time. Three, two, one—” Flexing their long toes the sisters sailed up into the air, and at the apex of their coordinated jump they popped out of existence.

On the sisters fled, to all the Nord’s many decks, shimmering into existence in parks, schools, museums, gymnasiums, theaters. In each place they stayed only a few seconds, just long enough to lock eyes, agree the next move, and jump or pirouette or somersault into it. It was really a kind of dance, the challenge being to control the accuracy of each Skim and the mirror-image precision of their positions and movements at each emergence.

Skimming, voluntary teleporting, was so easy small children learned to do it long before they walked. Alia’s body was made up of atoms bound into molecules, of fields of electricity and quantum uncertainty. Alia’s body was her. But one atom of carbon, say, was identical to another — absolutely identical in its quantum description — and so it could be replaced without her even knowing. She was just an expression of a temporary assemblage of matter and energy, as music is an expression of its score regardless of the medium in which it is written. It made no difference to her.

And once you knew that, it was easy to see that she, Alia, could just as easily be expressed by a heap of atoms over there as one over here. It was just a question of will, really, of choice, along with a little help from the nanomachines in her bones and blood. And very little Alia willed was denied her.

Most children Skimmed as soon as they found out they could. Adults found it harder, or gave it up as they gave up running and climbing. But few of any age Skimmed as skillfully as Alia and Drea. As the sisters passed, scattering startled birds, young people watched them with envy, and older folk smiled indulgently, trying to mask their regret that they could never dance so gracefully again.