Carefully, the mech said, "Well, the obvious to begin with. He ought to be able to buy new parts and upgrades as they come available. There should be ports and connectors that would make it easy to adjust to shifts in technology. He should be capable of surviving extremes of heat, cold, and moisture. And -- " he waved a hand at his own face -- "he shouldn't look so goddamned pretty."

"I think you look nice," the granddaughter said.

"Yes, but I'd like to be able to pass for flesh."

"So our hypothetical immortal should be, one, infinitely upgradable; two, adaptable across a broad spectrum of conditions; and three, discreet. Anything else?"

"I think she should be charming," the granddaughter said.

"She?" the mech asked.

"Why not?"

"That's actually not a bad point," the old man said. "The organism that survives evolutionary forces is the one that's best adapted to its environmental niche. The environmental niche people live in is man-made. The single most useful trait a survivor can have is probably the ability to get along easily with other men. Or, if you'd rather, women."

"Oh," said the granddaughter, "he doesn't like women. I can tell by his body language."

The young man flushed.

"Don't be offended," said the old man. "You should never be offended by the truth. As for you -- " he turned to face his granddaughter -- "if you don't learn to treat people better, I won't take you places anymore."

She dipped her head. "Sorry."

"Apology accepted. Let's get back to task, shall we? Our hypothetical immortal would be a lot like flesh women, in many ways. Self-regenerating. Able to grow her own replacement parts. She could take in pretty much anything as fuel. A little carbon, a little water ..."

"Alcohol would be an excellent fuel," his granddaughter said.

"She'd have the ability to mimic the superficial effects of aging," the mech said. "Also, biological life evolves incrementally across generations. I'd want her to be able to evolve across upgrades."