Within seconds he was swallowing the last of the leaf, savoring the lingering flavor on his tongue. It had been delicious but really quite light, and had done little but whet his hunger further. He looked around avidly. Here on the upper side of the treetop ceiling he could see the leaves turned downward toward the Quantum Sea, like a layer of broad, flattened child-faces. Farr reached down to pluck another leaf.

Dura, laughing, restrained him. "Take it easy. Don't strip the whole damn tree."

Around a full mouth Farr said, "It's delicious."

She nodded. "I know. But it won't fill your belly. Not unless you really do strip the tree... That's why we have to hunt the Air-pigs, who eat the leaves — and the grass — for us." She pursed her lips. Then, in a tone suddenly and, to Farr, shockingly similar to their lost father's, she said, "Let's have a little lesson. Why do you think the leaves are so tasty?"

Farr thought about that. "Because they're full of protons."

Dura nodded seriously. "Near enough. Actually they are laced with proton-rich isotopes — of krypton, strontium, zirconium, molybdenum... even a little heavy iron. Each nucleus of krypton, for instance, has a hundred and eighteen protons, while the tin nuclei of our bodies have just fifty each. And our bodies need protons for their fuel." The heavy nuclei fissioned in human stomachs. Protons combined with neutrons from the Air to make more tin nuclei — tin was the most stable nucleus in the Air — and gave off energy in the process. "Now. Where does the proton-rich material come from?"

"From the Crust." He smiled. "Everyone knows that."

The Crust, no more substantial than Air, was a gossamer solid. Its outermost layer was composed of iron nuclei. Further in, steepening pressures drove neutrons into the nuclei of the solid, forming increasingly heavy isotopes... until the nuclei became so soft that their proton distributions began to overlap, and the neutrons dripped out to form the Air, a superfluid of neutrons.

"All right," said Dura. "So how do the isotopes get all the way from the Crust to these leaves?"

"That's easy," Farr said, reaching to pluck another succulent leaf. "The tree pulls them down, inside its trunk."

"Using veins filled with Air. Right."

Farr frowned, feeling his cheeks bulge around the leaf. "But why? What's in it for the tree?"

Dura's mouth opened and closed, and then she smiled, her eyes half-closed. "That's a good question," she said. "One I wouldn't have thought of at your age... The isotopes make the leaves more opaque to the neutrinos shining out of the Quantum Sea."

Farr nodded, chewing.