I was taking a shortcut through the slave market, it not being an auction day, on my way to the jewelers' lane, when I saw that a sale was being offered and slowed down-a man who has been sold himself can't walk past, indifferent to the plight of chattels. Not that I had any intention of buying.

Nor did anyone seem about to buy this pair; the knot around the factor's tent was rabble; I could tell by their clothes and the fact that there wasn't a man there with a manservant.

The merchandise was standing on a table, a young woman and a young man. Late adolescence for him and just ripe for her, or the same age in view of the fact that females grow up faster. Call it eighteen measured by my own youth-an age at which a boy should be nailed into a barrel and fed through the bunghole but a girl is ready to marry.

Long sleeveless robes hung from their shoulders-and I knew too well what those robes meant; they would be displayed only to a prospective buyer, not to rabble. Robes signified valuable slaves, not to be knocked down on open bid.

Sure enough, they were being held at Dutch auction, with the minimum bid posted-ten thousand blessings. That amounts to- How can I define money of centuries back on a planet hundreds of light-years away in terms that make sense here and now? Let's put it this way: Unless these kids were something extraordinary, they were overpriced by a factor of five, as prime young stock, either sex, were fetching around a thousand blessings by the morning's financial news.

Ever pause in front of a clothing store and get hooked inside? No, of course you haven't. But that's what happened to me.

All I did was say to the factor, "Goodman, is that posted bid a mistake? Or do these two have something special that doesn't show?" Just curiosity, Minerva, as I neither intended to own slaves nor would the excess in my purse make a dent in a planetwide custom. But I could not see why? The girl was not outstandingly pretty; she would not fetch a high price as an odalisque. The lad wasn't even heavily muscled. Nor were they a matched pair. Back home I would have picked her for Eyetalian and him for a Swede.

Boom, I'm urged into the tent while the chattels are shoved ahead; the factor's manner shows that he hasn't had a live one all day-while my shadow is saying in my ear, "Master, that price is too high. I can take you to a private sale where prices are right and satisfaction is guaranteed."

I said, "Shut up, Faithful"-all rented body servants were named "Faithful," probably by contraries-"I want to see what this is."

As fast as the tent flap is fastened against the rabble, the factor is shoving a chair against my knees and handing me a drink with a bow and a scrape while saying lyrically, "Oh, sweet and gentle master, happy am I that you asked that! I am about to show you a great wonder of science! A thing to astound the very gods! I speak as a pious man, a true son of our Everlasting Church, one who cannot lie!"

A slave factor who can't lie has yet to be whelped. Meantime, the youngsters stationed themselves docilely on a display platform, and Faithful was whispering: "Don't believe a word, Master. The girl is nothing and I can whip three of that punk without my stick-yet the agency would sell you me for eight hundred blessings and that's a fact."

I motioned him to silence. "Goodman, what swindle is this?"

"No swindle, on my mother's honor, kind sir! Would you believe that these are brother and sister?"

I looked at them. "No."

"Would you believe that they are not only brother and sister but twins?"

"No."

"Would you believe the same stud, the same dam, the same womb, born the same hour?"

"Possibly the same womb," I conceded. "Host-mother?"

"No, no! Exactly the same ancestry. And yet-here is the miracle-" He held my eye and spoke in a hushed voice: "They are nevertheless a sound breeding pair...for these twins are unrelated to each other! Would you believe it?"

I told him what I would believe, including his losing his license and facing a charge of blasphemy.

His smile grew broader, and he complimented me on my wit and asked me how much-if he proved all of these things-how high a bid I would place against them? Higher than ten thousand since I must realize that the posted figure represented a prior bid. Fifteen thousand, perhaps, with escrow the morrow before noon?

I said, "Forget it, I'm shipping out before noon"-and started to stand up.

He said, "Wait, I beg you! I see that you are a gentleman of education, of science, of deep knowledge and widely traveled-surely you will grant your humble servant a moment to show proof?"

I still would have left; swindles bore me. But he waved a hand, and the kids dropped their robes and fell into display poses, the lad with his arms folded across his chest and his feet planted firmly, the girl in that graceful pose that must be as old as Eve-one knee slightly advanced, hand on hip, other arm hanging- easily, chest slightly raised. It almost made her beautiful save that she looked bored-having taken it hundreds of times, no doubt.

But that wasn't what made me stay; something annoyed me. The lad was bare of course-she was wearing a chastity girdle. Do you know what one is, Minerva?

"Yes, Lazarus."

Too bad. I said, "Take that damned thing off that kid! Now!" Silly of me; I rarely interfere with anything on a strange planet. But those things are abominations.

"Certainly, gentle sir; I was about to. Estrellita!"

The girl turned her back, with that same bored look. The factor stood so that his back kept the lad from seeing him work the combination lock, saying apologetically, "She must wear it not only because of ruffians but to protect her from her brother; they share the same pallet, for she is-would you believe it, sir, seeing how full ripe she is?-a virgin! Show the gentle master, 'Trellita."

Bored as ever, she promptly started to do so. I regard virginity as a correctable perversity of no interest; I motioned her to stop and asked the factor if she could cook.

He assured me that she was the envy of every gourmet chef on Blessed, and started to lock her back into that steel diaper. I said roughly, "Leave it off! Nobody here is going to rape her. What's this proof you promised?"

Minerva, he proved every word-except about her cooking-with exhibits that made me suspicious only because he showed them; I wouldn't have boggled had I seen them in the Clinic here.

I should mention that Blessed had a rejuvenation clinic even though it was not settled by the Families. Eventually the clinic was taken over by the church and antigeria techniques that work fairly well even on short-lifers were no longer available to any but big shots. But the planet stayed advanced in biological techniques; the church needed it.

Minerva, I told you that he claimed and you are now as learned in biology and genetics and associated manipulations as Ishtar is-more so; you don't have her limitations in time and in memory storage. What did he prove to me?

"That they were diploid complements, Lazarus."

"Right! Although he called them "mirror twins." Can you tell me how these kids were made, Minerva? How would you go about producing such twins?

The computer answered thoughtfully, "'Mirror twins would be an inexact term for zygotes satisfying the listed requirements-although it is colorful. I can answer only theoretically as the records in me do not show that it has been attempted on Secundus. But the steps necessary to achieve exact diploid complements would be these: There must be intervention in gametogenesis in each parent just before meiotic division-reduction of chromosome number-that is, one would start with primary spermatocytes and primary oocytes, unreduced diploids.

"In the male parent the intervention presents no theoretical problem but would be difficult because the cells are very small-but I would not hesitate to attempt it given time to construct the necessary fine extensionals.