Warning: Contains some spoilers for Full Metal Alchemist.

XXXchange
If there was anything Watanuki hated the most of working for Yuuko, it was cleaning her shop. He could accompany her on her strange adventures or cook for her, Maru, and Moro, but he hated cleaning shop. He didn't mind the chore itself so much, but it meant that he had to spend even more time around the creepy items she'd collected over the years.

He sighed as he walked up to the first shelf, cleaning supplies in hand, muttering, "Better get this over with."

The cleaning itself was easy, and he was glad to see that not all of the items Yuuko had gave him the chills. Even so, he still felt uncomfortable knowing that she'd taken most of it as payment for her services. He'd been so unnerved at the sight of a broken old locket that he dropped the sword he had to polish next, the one that she'd taken from the ninja Kurogane. He quickly stumbled backwards to avoid stabbing his foot and crashed into a large sheet-covered artifact behind him. Years and years of dust flew into the air from his disturbing the sheet, so he carefully pulled it down to keep any more from being released. What he thought might be a mirror turned out to be a large pair of doors—a gateway of some kind. Watanuki felt sick as he stared at it, and with his cursed sight, he could see shadows trying to pry the doors open from the inside. He had a sudden urge to grab Kurogane's sword and try to fend off whatever power was emanating from that gate.

"There's nothing to worry about, Watanuki. The Gate's sealed tight." Yuuko announced, walking up to him. She placed a hand on the Gate and smiled. "Nothing's going to be coming out of here for a long time, unless someone does something stupid again."

"What?" Watanuki croaked.

"Humankind cannot gain anything without first giving something in return. To obtain, something of equal value must be lost." Watanuki gave her a confused look—why was she talking about her shop's policy at a time like this? But she smiled and added, "That is alchemy's first law of equivalent exchange."

"Equivalent exchange?" he questioned.

"Precisely. There are some worlds that have people capable of using alchemy—the transmutation of some materials to others—and one world in particular has many alchemists. Their alchemists became so curious about the science that they began pushing its boundaries, so finally, laws had to be written. Equivalent exchange is one of the most important, and almost all the alchemic laws come back to it."

"What does this Gate have to do with it?"

"Once, two very young alchemists decided to break one of the laws and tried to bring their mother back from the dead. The older brother lost his leg in the attempt, and the younger nearly lost his life. The older brother was brought before the Gate, and it showed him everything before sending him back. When he discovered that he'd lost his brother, he came to the Gate again, asking for his brother's soul back. I gave him a very good deal for it: just an arm."

"You call that a good deal?" Watanuki sputtered, momentarily ignoring Yuuko's admission that she guarded the Gate.

Yuuko dropped her smile and gave him a terrifying glare. "It cost one child's body and soul and another's leg to try and bring back the mother. Considering what it could cost to bring back a soul—something forbidden to exchange in the laws of alchemy—I think an arm is a very fair deal, don't you agree, Watanuki?" He could only nod.

"But what happened to their mother? Were they able to bring her back?"

"No," Yuuko answered without sympathy. Those boys had ignored all the laws, and they had to pay for it. They were only lucky that she had somewhat of a soft spot for children. "They learned a valuable lesson about defying the alchemic laws, even if they were determined to restore their bodies with the Philosopher's Stone."

Watanuki noticed that the longer Yuuko stood by the Gate, the less the shadows tried to escape. "Is that thing safe?" She patiently looked at him for clarification. "There are shadows trying to come out of it—less now that you're standing there, but…"

"You have no alchemic power, so you can't draw them out," she answered. "Those are homunculi, soulless human-like creatures created from human alchemy, when someone tries to resurrect the dead." Watanuki suddenly thought of Maru and Moro before a key part of the explanation hit him:

"Wait, from resurrection? Then those boys created…"

"You'd better finish cleaning up," Yuuko suggested. "We've got another customer to take care of." She started to walk away, but stopped and added, "And leave the Gate uncovered. It really does look quite beautiful, despite its reputation." And with that, she left.

Watanuki hurriedly finished that corridor and threw away the sheet that had covered the Gate, swearing to avoid cleaning it again for as long as humanly possible.

I don't own XXXholic or Full Metal Alchemist. This is just a crack fic that I came up with while talking to my sister.