Sheldon was once again heckling Penny on her relationship with Leonard, causign her to finally burst out,
"Because I love him Sheldon!"
His expression did not change.
"I see, now when you say "love", exactly what do you mean by that?" he asked casually, taking up a clipboard.
She hesitated.
"Well, I - I don't know, I - it's not something you -"
"If I may interject," he said with his normal blatant disregard for most of what she Penny said, "all words have definitions, but some, as I am led to believe, have certain meanings to certain people, and as you seem to be capable if love, I am interested in what your definition is."
"You are?" her eyebrows shot up in disbelief.
"Well, yes, of course I am not actually interested in your opinion, but in the purely scientific observation that I would be conducting in asking you to describe something that is not easily described."
She gave him a look.
"And also," he went with slight hesitation, "Amy and I have been going out for some time now, and naturally over the next, say ten or fifteen years, she might expect me to express my feelings for her, and I want be sincere. Just as Leonard demanded of you, now, if you're quite done understanding the observation I am trying to accomplish would you please answer the question."
"Uh, yeah," said Penny, a little dazed, "what was the actual question sweetie?"
He sighed.
"I shall reiterate it for you, perhaps in a way your uneducated left cerebral cortex might better comprehend, though I thought what I said before was quite simple."
"Sheldon…" she warned.
"Fine, how do you know you love Leonard?"
"Sheldon, I told you it's not something you can really explain."
"Well of course it is, almost everything that happens can ultimately be explained, though any theorist would have his work cut out for him as to why you Leonard continues to stay with you."
"Sheldon, if you want my help, you have to be nice to me."
He looked confused, but continued anyway.
"Try penny... please," he added rather uncomfortably.
"Uh okay," she paused for a moment and then said slowly,
"I love Leonard because... Well, because he makes me laugh... And I think it's so adorable when he's afraid that I'm mad at him, or that he writes letters to me thanking me for having sex with him, and I love that when I try to act smart he nods and pretends I'm making sense or the fact that he knows he could do better and get someone smarter but he wants me, and I never want to be with any other man ever, even when he frustrates me or makes fun of me I know that he would do anything for me, and I can never really be mad at him. He's so different than what I expected my husband to be like, so much better and as much as I've changed him I know he's changed me too, and I thank god that he has. Seeing his face makes me smile like right away, I can't help it. He screws up sometimes but I know that he would never in a million years hurt me, and that he would face any alien or bully or enchanted warlock thing if it meant keeping me safe. He's the type of guy that only comes around once in a lifetime, and I'm just so glad that he's that guy. Just him Sheldon, everything about him."
"Hmmmm... I see a pattern," said Sheldon, scribbling on a clipboard, "laughter, small acts of kindness, desire naturally, forgiveness, protection, and seemingly inexplicable happiness that he is "that guy", am I correct?"
He didn't wait for her response.
"Oh what am I asking you for of all people of course I'm correct."
She let out a sigh.
"Oh Sheldon sweetie, you have a lot to learn about love. Why do you have to take notes, don't you have an eidetic memory?"
"Oh yes it's quite extraordinary, but I am taking extra precaution as this is something I really don't want to forget."
Penny looked at him with stunned compassion, her eyes softened.
"Aw Sheldon, you are human after all…"
He looked up, evidently annoyed.
"Oh yucky what makes you say that, what did I do?"
Penny just chuckled and shook her head.
"Nothing, Sheldon, nothing."