A/N This is an answer to deelaundry's fic challenge on Valentine's day on her lj. I'm not sure I got this right but I tried, inspired by black_cigarette's "Wilson wouldn't be able to prove he did". I like proving things. I also made a change in my original response inspired by a comment by deelaundry.
The prompt was: in episode 7-12 of House, "You Must Remember This", House gave Wilson ten days to "get back into it." What happens when the ten days are up, if Wilson doesn't?
"It's nine days. How is it going?"
"Not ten yet. Also, we haven't made any concrete bet."
"You mean it? Ok. So, if you don't, you get rid of the cat. Find some shelter or whatever, just get rid of it."
"Fine. And if I do, you move back in with me for a week."
"What? With the cat?"
"If I can prove it, remember."
"It's a bet."
"See you tomorrow at 9 at my place."
"Here's your proof. Come back tomorrow evening, this time, to admit defeat. With your suitcase. I'll order Chinese."
House opens carefully the refrigerated box, peers inside, and closes it.
"You want me to analyze... those?"
Wilson looks totally innocent, while Sara purrs satisfied in his arms.
"Inside and outside. Here, throw that into the mix." He pulls out a cotton swab, slides it deeply into his own mouth, puts it into a plastic bag, closes it, and adds everything to the refrigerated box. "Goodnight, House."
"Hi! Nice to see you have brought your stuff. If you forgot your hair drier I can lend you mine."
"One condom with female saliva outside, your sperm inside. Another with vaginal secretions outside, your sperm inside. Sperm identified as yours via the cotton swab. Both female samples come from the same woman. Where's the cat anyway?"
"My neighbor had left her to her grandson. He had a job trip lined up so he left the cat with me. He got her now, but I promised to cat-sit again in the future if need arises." Wilson smiles happily and sits down on the couch. "I've TiVo'd your favorite Soaps. You neglected them, too."
A week later House and Wilson are chatting on their common balcony after lunch.
"It was great having you living with me again! Worth the effort to win the bet."
"It was a win-win-win for me; you got laid, the cat's gone, and I spent time with you. I had missed you." House pauses and thinks.
"Maybe I could have a similar bet with Cuddy and have her get rid of Rachel instead of the cat."
"What if she wins?"
"Come on, be serious. She's a full time hospital administrator, the mother of a toddler and has an insatiable lover. When would she have time to win such a bet?"
"Have you tried putting a cotton swab in her mouth?"
House's eyes widen. He limps top speed out of the balcony and out of his office.
House drops the cane, grabs Wilson by the lapels of his lab coat, lifts him and bangs him against the nearest wall - as it happens, the one dividing their offices.
"You traitor! Snake! How could you do that to me?"
Wilson tries to breath, but can't. The best answer he can manage is a silent "mmmhhhh".
House notices the problem, so after banging him against the wall another couple of times, thus ensuring at least a concussion and the careful interest of all the ducklings, he lets him stand on his feet.
"Don't go all Othello on me. I did what you told me to do."
"You slept with my girlfriend and that's all you can say?"
House looks ready to punch Wilson in the face, but stops when he notices no sign of defense, or apology. He then slowly slides down to the floor, his right leg straight in front of him, his face in his hands. He's crying.
"I'm sorry. Stop that. Joke is over." Wilson bends down and puts a hand on House's shoulder, which House promptly scoots away. He has stopped crying and seems ready to hit again.
"I didn't sleep with your girlfriend. I asked a med school friend who does research on peri-menopause to collect samples and questionnaires at PPTH. She then reported Cuddy's samples as lost and handed them to me."
Wilson hands him a glass of liquor, which House gulps down.
"I'll get back into it when I feel like it. I wouldn't have believed you could be such an idiot."
House remains on the floor, now crying for a different reason.
A/N In case this wasn't clear: Wilson thought that knowing that the female material was Cuddy would convince House that the whole thing was a prank, maybe organized with Cuddy's assistance. House finally cries because he realizes he doesn't trust even the two people who are closest to him. He has no deep human relationship and it's all his fault.