Summary: Three times Sheldon kissed Penny (although the first one doesn't really count)
The first time Sheldon kisses her doesn't count, because Penny tricked him into drinking by saying, "Yes, Sheldon, of course these are virgin daiquiris" when they really weren't. Call it a social experiment, call it mean, but it was definitely funny. She justified herself later by saying that he only kissed her cheek and they were in his apartment, so the only witnesses were Leonard and Raj (Howard had left an hour earlier mumbling something about an early project at work the next day). They played a Wii tennis tournament, Penny and Sheldon teamed up against the other two, and their team of awesome kicked the others' asses, crowing their victory loudly and triumphantly. Well, Penny had crowed, Sheldon had just talked of their superiority loudly.
His inhibitions must have been completely nil because while they were doing their victory dance around the living room, Sheldon ended up with his arm over her shoulder and of course she leaned in and then—well, it was over pretty quickly when the alcohol caught up to him and he ran to the bathroom to be violently sick over the toilet.
So not really a first time, but it did bring back memories of Leonard's reaction to too much schnapps. Poor bastard.
This time, however, they're alone and Penny is rolling her eyes at him because he's making fun of her again and not realizing that he's also leaving himself open to attack because he doesn't understand sarcasm, like at all.
"So you're infallible?" She vaguely knows what the word he used means, but more importantly knows that Sheldon thinks he's untouchable and never ever wrong. "You never make mistakes?" She's sitting next to him on the couch, in the middle seat, with her leg almost touching his. "You're never open to suggestion?"
Of course he misses the point, goes off on one of those tangents he likes, spouting names she doesn't know, but assumes are the scientists and mathematicians he lusted over as a young child (and might still to this day). He's waving his arms in the air and opening his mouth to tell her again just how wrong she is when she leans over and stops his tirade with a kiss.
He freezes under her and her heart skitters, scared. Wait, what is she doing again? She starts to move back and apologize when his hand comes to rest on her shoulder and he pushes her more firmly into him. Oh. Oh. He's kissing her back and she's trying not to smile against his lips because this is Sheldon for God's sake—they're both sober so what is he thinking? But then she's straddling his lap and they're sharing his spot for once and it's good, so good. He's muttering against her neck when they come up for air and she wonders if he's continuing the argument even with a lap full of willing female, which is such a Sheldon thing to do. Penny doesn't find that cute. Of course she doesn't.
She does what any normal woman would do when faced with a man who's is thinking too much and kisses him again, clenching her fingers in his hair so that she can mess it up even a little and prove that hell yes, this actually happened.
Because she hears a key in the lock, she scrambles off of him just when her breath is starting to shorten and they were both getting way interested.
"Point proven," she says loudly and pokes him in the chest just as Leonard walks in.
"What point is that?" Leonard asks, throwing his keys into the bowl and sitting in the open chair.
"Only that Sheldon has weaknesses."
Leonard's eyebrows shoot up, especially when Sheldon only glares halfheartedly at Penny and says absolutely nothing in response.
The third time Sheldon kisses her they're alone. Just when her neck starts to heat up and she starts to get angry in the middle of their argument, he pulls her to his room. This time, there is no chance for interruptions and, even though she would love to dirty that spot on the couch, she's grateful to have him all to herself.
"Tell me again why you think I'm wrong," she says later,drawing patterns on his chest and loving that he's shivering in response.
"Well you see, Penny, it's very simple—"
Penny is proud that she lets him talk for a full minute before kissing him again.