“Now there’s just the two of us.” Mary Mother-of-Mac sat down beside me. “So, my dear, why don’t we find out a little more about each other?”

I learned during the next three-quarters of an hour what she meant by that. I was asked a series of penetrating questions regarding everything from my education and job description to my personal hygiene and tastes in men.

At the end of it she sat back and gave me a big smile. “You know, that is so much a relief. Artie is such an innocent. I was afraid that he might have fallen for a pretty face.” She thought for a moment, possibly decided that she was being less than tactful, and amended her statement. “Or he might have found an intellectual. That would be even worse.”

I said, “Perish the thought.”

It was wasted on her. “Now I’ll tell you what’s happening and why I’m here,” she said. “First, I’m going to be married.”

I made conventional sounds of congratulation.

“Well, I mean, it’s as good as being married. Fazool and I are going to live together. He’s enormously rich, and he likes the idea that I’m utterly poor. It makes him feel protective — he thinks if it weren’t for him I’d be in the poor-house.”

The house I would suggest for her sounded rather like poor-house; but I kept my mouth shut.

“Fazool would be very upset,” she went on, “if he ever found out that I had funds of my own. So I’ve decided to put my money into a trust. Artie is my only child, and the lad will be the ultimate beneficiary. I’m glad you’re around to take care of him, because he can be such a dim-wit.”

I looked around. The tea-room would be filling up in a few minutes, but fortunately the place was still deserted except for the two of us. Describing McAndrew as a dim-wit at this Institute would get you the same reaction as chug-a-lugging the altar wine during a church service.

“What about Mac’s father?” I asked. “Shouldn’t he be a beneficiary?”

“Ah, yes.” Her face took on a look of wistful sadness.

“Dead?” I realized that I had never heard McAndrew speak of his father, not even once.

“By all the logic, he is.” She smiled sweetly. “But a son-of-a-bitch like that is awful hard to kill.”

The arrival of a chattering half-dozen scientists saved me from fielding that remark. Mary McAndrew made an instant survey, checked the line of her skirt to make sure that plenty of leg was showing, and headed for the tallest and most distinguished-looking of the group. It was Plimpton, who according to McAndrew had not had an original thought since he started to grow facial hair and possibly not before. On the other hand, I don’t think Mary was seeking original thought. Original sin, maybe.

I followed her toward the tea and sweetmeats. Apparently I had been weighed in the balance and found reasonably adequate. But I suspected that Mary McAndrew employed an unusual scale.

A mother, and now a father, too. I couldn’t wait to hear McAndrew’s side of the story.

But wait I had to. McAndrew arrived at last from the seminar with half a dozen other scientists. He headed toward his mother. Before they could exchange more than two words, Emma Gowers came sashaying over toward them.

A word about Emma. She is the Institute’s expert on multiple kernel arrays and a formidable brain. She is also blond and beautiful, with a roving eye, a lusty temperament, and a taste for big, hairy men of diminished mental capacity.

I was standing only a step away. I saw Mary McAndrew and Emma size each other up, and I realized that neither knew who the other was. But like called to like, and they straightened and preened like two fighting cocks.

“Come on, Mac,” Emma said. “You and I have a date.”

The wording was provocative, but I knew that Emma had no possible sexual interest in McAndrew. His mother didn’t. So far as she could tell, Emma was cutting in.

“I beg your pardon?” she said.

McAndrew made a feeble gesture from one to the other. “Mother, this is one of my professional colleagues, Emma Gowers. Emma, this is my mother.”

Mary McAndrew extended a slim and delicate hand. “And which profession would that be, my dear?” Her tone couldn’t have been warmer.

Emma gave her a friendly smile. “Not the one you are most familiar with, I’m sure.” She had been making a close inspection of Mary McAndrew’s neck and the wrinkles at the corners of her eyes. “But it’s encouraging to know that a person doesn’t have to change her line of work, just because she’s old. Come on, Mac.”

She gripped McAndrew firmly by the arm and pulled him away toward the door. I was left to face his mother.

I said, “It’s not the way it looks. She’s not chasing him. There’s a problem with the balanced drive on one of the ships, and he and Emma have an appointment to take a look at it.”

Mary McAndrew seemed not in the least upset. She said thoughtfully, “Well, I certainly underestimated that one. She and I must have a cozy chat when they get back. Where do you say they’re going?”

It was easier to show than to tell. I put down my cup and led her across to one of the room’s small observation ports. “They’ll be going outside the Institute and over to one of the ships. You can see it from here. That’s the Flamingo, the Institute’s smallest experimental vessel.”

She followed my pointing finger. The Flamingo was berthed about four kilometers away. We had a profile view of the circular flat disk of condensed matter at the front, with the long column jutting away from the center and the small sphere of the life capsule sitting out near the end of it.

“What a strange-looking object!” Mary said. “Why, it’s not in the least like a ship.”

I stared at her. Was she joking?

“You’re looking at a ship that uses the McAndrew balanced drive,” I said. “Mac says it’s a trivial idea, but it’s the most famous thing he’s ever done. He’s known everywhere in the Solar System because of it.”

“Is he now?” She peered at it with a bit more interest. “But it’s ugly. That plate, and the long spike. And where do the people sit?”

She didn’t know, she really didn’t. Her own son’s most celebrated invention, and she had no idea.

“The crew and passengers go in the life capsule.” I pointed. “That’s the little ball you can see at the end of the spike.”

“But it’s teeny. All that big ship, and such a small space for people. What a waste.”

“It has to be that way. That plate on the front is a hundred-meter disk of compressed matter, electromagnetically stabilized. If you put people in the middle of the disk while the ship is at rest, they’d feel a gravitational pull of fifty gees — enough to flatten anybody. But in the life capsule out at the end of the spike, a person feels a pull of just one gee. Now when you turn the drive on and the acceleration grows, the life capsule automatically moves closer to the disk. The acceleration and the gravitational force pull in opposite directions. The life capsule position is chosen so the total force inside it, the difference of gravity and acceleration, stays at one gee. A lot of people call it `the McAndrew inertia-less drive,’ but Mac hates that. He says inertia is still there, and the right name is the balanced drive.”

I should have more sense. Predictably, I had lost her. In the middle of my explanation she had turned away from the window and she again had her eye on the mentally nulliparous Plimpton.

“Gravity, acceleration, compressed matter,” she said. “Oh, how that carries me back. Like father, like son. McAndrew’s father, he’d drive a woman mad with talk of compressed matter, when what she was needing was a little personal attention.”

“McAndrew Senior was a physicist, too?” If I couldn’t get family information from Mac, maybe his mother would provide it.