It happens gradually.
Penny's explanation of why Sheldon's spot is Sheldon's spot, Penny's scathingly Sheldonized version of Leonard's experiment. It's amusing, really, and Leonard doesn't really consider it. Doesn't really consider the implications behind it.
Three Weeks Later.
Sheldon and Penny are watching Mr. and Mrs. Smith. She'd mentioned they were going to earlier, something about payback and emergencies and an invitation for Leonard that he'd declined because of work and the explosion that would no doubt be caused by Sheldon watching it.
Except. They're sitting, watching, and Sheldon appears to be neither scoffing nor unduly suffering, and Leonard pauses as he walks behind the couch.
"Enjoying the movie?" he says, smug as all get out, but Sheldon doesn't so much as glance away from the screen.
"Despite the fact that Angelina stole Brad from Jennifer, Penny is right, they do have vast amounts of "chemistry" in this movie," he says, voice low as he watches the title characters fight each other. Penny looks over at him, real pleasure and pride in her face as she pats his arm and then glances up at Leonard as if to say see? Leonard realizes that he's gaping like a fish, so he smiles back at Penny and keeps walking. Clearly this is simply an aberration.
Two Weeks Later.
Penny pauses outside the restaurant with a little frown, and Leonard pauses besides her. "What's wrong?" he asks.
She shrugs a little, trying to ignore the odd feeling.
"Penny?" he prompts again, and she shakes her head.
"It's just…it's Thai night. And we're getting Italian."
"You don't want Italian?" Leonard asks, confused, and she shrugs again, evidently bewildered by her own thoughts.
"It's just…it's Thai night," she says, as if it's self-explanatory. And in a way, it is.
Seventeen days later.
"Howard, given the current societal trends in fashion, should you not rethink your choice of dress if you are, in fact, hoping to attract females rather than cause them to run and/or hide at your very approach?"
"What?" Howard asks, more at the content rather than the delivery of the words, but Sheldon sighs over-dramatically.
"Given the magazines Penny has perused in my company, I have noticed that your choices in color as well as combination leave much to be desired. If you let Penny give you a "make-over," I am confident that your desirability to the female population would increase—that, in short, you'd 'get a lot more babes.'"
"Sheldon, are you giving me dating advice?" Howard asks, trying for a smirk.
"Howard, I think it's clear to everyone that someone needs to," Sheldon deadpans.
One Month Later
"This is ridiculous," Penny vents to the boys. "Everyone is calling in sick, the virus is spreading like tribbles and production is going to be completely shut down if this keeps up!" She blinks, noticing that they're staring at her, most of them trying not to grin. "What?" she snaps.
"Like tribbles?" Howard asks amused, and she rolls her eyes.
"Given my temper right now, Howard, do you really feel that it's in your best interest to aggravate me in such a way?" she says, eyes flashing, and then she claps a hand over her mouth, eyes wide.
"What?" Leonard chokes out, shocked, and she shakes her head, trying to breathe calmly.
"I mean, shut up before I make you?"
One Week Later
"Perhaps I am not making myself clear," Sheldon says, heat in his voice that no one is used to. "Penny is dating my friend Leonard. Despite the fact that I am certain this relationship is doomed to failure, this does not negate the fact that Penny is currently involved with someone. Penny is also my friend, and as she has told you that not only is she dating someone, but that she would not be interested in you if you were the last person on the planet, I believe it is time you admitted your defeat and began to pursue someone else."
"Listen, pal, this isn't any of your business, so keep walking before I feel the need to convince you," the other man says, starting to step around Sheldon to a fairly pissed-off Penny. Except Sheldon, inch-for-inch as tall at the other guy, steps in his way.
"Let me be even more clear," he says, voice gravelly with irritation. "Get. Fucking. Lost."
Something about the delivery and the way that Sheldon and Penny are both shooting him identical death glares must get to him, because the guy stands his ground for a long moment and then with a decent amount of cussing stalks off. Penny just smirks a little at Sheldon as Raj and Howard stare at them with flabbergasted expressions.
Two Months Later
"Okay, Leonard, here's the thing. It's been dead. All this time. And we've just been pretending that it's alive, and I can't pretend anymore."
"Penny, I don't understand what you mean!" Leonard yells, furious and frustrated by now, and Penny throws her arms up, exasperated.
"We gave it a shot, okay? But just because we pretend that we're compatible, doesn't mean that we are! Schrodinger's cat isn't opening the box and seeing if the cat is alive or dead, it's the fact that the cat's alive or dead without the universe's say-so or seeing, and no matter how much we say we work it doesn't change the fact that what's happened in the box has happened, with or without our consent!"
"You're breaking up with me," Leonard says, voice stiff, stilted. "You're breaking up with me, and your reason is Schrodinger's cat?"
"We don't talk," she says. "We don't fit," she says. "This was inevitable, and you know it, Leonard."
"You sound like Sheldon," he says, and now he sounds bitter. "Eight months of a relationship and now you're telling me we were never together? What the fuck, Penny?"
"Leonard," she says, and her voice is shaky with unshed tears, and she catches his tense fingers in her hands, pulls his arms from their defensive position crossed in front of his chest. "Leonard," she says. "'Two roads diverged in a yellow wood.' We were together, Leonard. For what it's worth, we were together, and we tried, and I wanted to believe. I'm glad I took this road, I'm glad I believed we could work."
He breathes out, closes his eyes for a long moment before letting them open. "But we couldn't," he says. "We can't."
"Not like this," she says. "This isn't us. This isn't who I want to be."
He nods, with something like a sigh on his lips, and then he pulls back and walks away. He doesn't let himself look back.
Eighteen Months Later.
"Sheldon, this is a non-optional social convention," Penny says, trying to hide her grin.
"Penny, I know that you are simply using that phrase to elicit my consent, and—"
"Think of how you'll be able to keep my apartment clean all the time," she says, amused, and Sheldon pauses, a considering look on his face before he shakes it away.
"Penny, despite our relationship I know that my…quirks, as you call them, often cause you to feel irritation, and you moving in would—"
"Leonard is moving in with Leslie, and it—" she tilts her head to the side, pausing a moment to choose the right words, "It would not only be more practical but also beneficial for us to consolidate our resources and share the apartment."
"Penny…" Sheldon says, still looking nervous at the thought, "I don't…I don't want to lose you. And I think…I think that you having to put up with me…"
She puts a hand to his cheek, and he stills, looking down at her with so much warmth Leonard can feel it radiating from where he's paused, frozen, in the doorway.
"I'm not going anywhere," she says. "I promise."
"I love you," Sheldon says, the irrational, emotional, ridiculously sentimental and completely un-Sheldon words flowing easily off his tongue as if he were born to say them, and born to say them to her. "I love you, Penny. Move in with me."
"If you insist," she laughs, and then she kisses him, and Leonard shuts the door quietly, backing away from their perfect tableau.
Leslie, after all, is waiting for him, and even he can see what's happening inside the box.
Much love, Wolfie ;)