Scratching at his beard, the Russian explained, "Each morning brings a new ache. My eyesight isn't what it used to be. My prostate is growing."

"That is natural," said the doctor.

"Yes, but my nanotech friends tell me that they could bring my eyesight back to twenty-twenty and shrink my prostate back to normal and strengthen my poor old muscles, with nanomachines."

Joanna looked at her husband with new eyes. Lev had never complained, she had never had an inkling that he felt his years. In bed he was as vigorous as men half his age. But if he feels old and creaky on the Moon, he must be in agony now, here. Yet he won't show it, not even to me.

She reached out and grasped his hand. He looked surprised, then grinned sheepishly at her.

The doctor was oblivious to their byplay. He said to Joanna, "But you, Mrs Brudnoy, you have used nanomachines, haven't you?"

Joanna nodded easily. "Many times. For cosmetic reasons, mainly, although I've had them scrub plaque from my arteries more than once."

"You see?" the doctor said, as if she had just confessed to a crime. "We cannot take the risk of having nanomachines infect our terrestrial environment."

"Doctor, I'm surprised to hear such nonsense from an educated man," Joanna said.

"Nonsense?"

"Of course it's nonsense. To begin with, there are no nanomachines in me. I underwent therapy and then the machines were flushed out, quite naturally."

"How can you be sure-"

"They know the number of machines they put in," Brudnoy explained, "and they count the number that come out. It's quite simple."

"But they could multiply inside the body, couldn't they?"

"Only if they're programmed to do so," said Brudnoy.

Before the doctor could reply, Joanna went on, "Second, and more important, is that nanomachines are machines. They are not alive. They cannot mutate and change. What if there were a few nanomachines left in my bloodstream? What harm could they do, even if they got loose into your environment?"

"That depends on what they were designed to do, I should think."

"Yes.' Joanna's smile returned. "If a few got into you, for example, they might remove some of the fat you've accumulated."

For an instant she did not know how the doctor would respond. He stared at her as he digested her words. Then his round pudgy face opened into a hearty laughter.

"Nanomachines could make me slim!" he gasped. "I could eat whatever I like and still become thin!"

Joanna leaned back in the stiff metal chair, thinking that she had won the man over.

But his laughter died away. "All this may be very true, but suppose you are carrying nanomachines that are harmful?"

"Harmful?"