...Actually it was a long time since he'd done this, he realized a few heartbeats after starting. His palms grew slick with superfluid sweat, a steady ache spread along his back, and for some reason the vision of his good eye was starting to tremble and blur. And, though the spear bowed upward a bit as he strained, the branch did little more than groan coldly.

He let go of the spear and wiped his palms against his thighs, feeling the breath rattle in his chest. He carefully avoided eye contact with the boy.

Then he bent to the spear again.

This time, at last, the branch gave way; a plate of it the size of his chest yielded and lifted up like a lid. Adda felt his aching legs spring straight, and he tumbled away from the branch. Quickly recovering his dignity, he twisted in the Air, ignoring the protests from his back and legs, and Waved back to Farr and the opened branch. He looked down at his handiwork appraisingly and nodded. "Not as difficult as it looks," he growled at the boy. "Used to do that one-handed... But trees have got tougher since I was your age. Maybe something to do with this damn spin weather."

But Farr wasn't listening; he crept forward to the wound in the branch and stared into it with fascination. Close to the rim of the ripped bark the wood was a pale yellow, the material looking much like that of the spear Adda had used. But further in, deeper than a hand's length, the wood was glowing green and emitting a warmth which — even from half a mansheight away — Adda could feel as a comforting, tangible presence against his chest. The glow of the wood sparkled against Farr's face and evoked verdant shadows within his round eyes.

Dura, Logue's ungainly daughter, joined them now; she shot a brief smile of thanks to Adda as she crouched beside her brother and raised her palms to the warmth of the wood. The green fire scattered highlights from her limbs and face which made her look, Adda thought charitably, half-attractive for once. As long as she didn't move about too much and reveal her total lack of grace, anyway.

Dura said to Farr, "Another lesson. What's making the wood burn?"

He smiled at her, eyecups full of wood-glow. "Heavy stuff from the Crust?"

"Yes." She leaned toward Farr so that the heads of brother and sister were side by side over the glowing wood, their faces shining like two leaves. Dura went on, "Proton-rich nuclei on their way to the leaves. The tree branch is like a casing, you see, enclosing a tube where the pressure is lower than the Air. But when the casing is breached the heavy nuclei inside fission, decaying rapidly. What you're seeing is nuclei burning into the Air..."

Adda saw how Farr's smooth young face creased with concentration as he absorbed this new bit of useless knowledge.

Useless?