Sheldon sees how much fun Leonard and Amy are having in S11Ep13 and he gets kind of jealous.
"This one was under the purple light," Leonard says as he moves the small box toward Amy, and she measures the growth of the bean shoot in it.
"As expected, this is smaller than the beans under the red and white lights, but it does have larger leaves," Amy observes. She smiles, and looks over to another box, "This one instead was under the white light, and grew bigger and healthier than all the others, included the control sample we left near the window," she concludes, pointing to a third plant there.
Leonard nods, "That's right," he confirms. They exchange a proud stare: even if it's a fifth grade kind of experiment, it feels good it turns out as expected.
It's already two weeks they meet almost every night to recreate old science fair projects. At first it was because Sheldon wanted to be alone to focus again on string theory and kicked Amy out of their apartment, but the routine has continued even after he moved to his old room. As for Penny, she has hung around the first week, until she has signed up for a refresher course.
"This has been so much fun," Amy says, clapping cheerfully and awkwardly, "It totally made up for when I first tried this experiment."
"Why, what happened then?" Leonard asks.
"My cousin replaces the bean seeds with marijuana ones. Oh, don't worry," she reassures Leonard when she sees his concerned look, "I realized it before my bedroom became a crime scene."
"Wait, it's the same one who your mother paid to bring you to prom and instead used the money to buy drugs?"
Amy is surprised. She has maybe mentioned it many years ago, so she's pleasantly surprised that Leonard remembers that, "Yes. And to anticipate your next question, he's the one sending those lovely Christmas Cards every year from the Glendale City Jail."
"Well, I assure you these are absolutely beans," Leonard comments, "I'm sorry it went that way back then," he adds then with a gentle smile.
Amy shrugs, "It's okay. At least he was always around to check on those plants. I didn't get to participate in that science fair, but for some weeks I had someone entering my room who wasn't my mother."
Leonard looks back at her. She's now duckling away, and he knows it's because recalling how lonely her childhood was is not so great. He knows that because he feels the same.
"You know, it sucks you were so alone growing up, I bet you were an amazing kid," Leonard tells her, "Brilliant and funny at least, seeing how you are now." His face also tells he really means that.
She blushes, "I bet you were too. It would have nice, if we had met back then."
"But wait, you have siblings," Amy notes, "And still you were always on your own?"
"They are older, you know," Leonard explains, "And they are truly our mother children."
Leonard sighs, and reflects a moment, "You know," he confesses, "I have always wanted to have a little sister, possibly who could share some of my interests. We could have stayed up together at night chatting, and gotten into endless tickle fights."
"You know," he adds after a brief reflection, "You would have been an amazing sister to me."
Amy feels a little embarrassed, but overall flattered. It sounds kind of odd, mostly. Not bad, though.
"Thank you, then," she agrees. Still, a question remains.
"Why a younger sister and not another brother?" She asks. Leonard grimaces, isn't it obvious? "Really? Boys are animals. Tickle fights turn into wrestling in no time."
Amy doesn't agree, "Are you sure? I bet your little sister would have kicked your ass like nothing," she jokingly provokes him.
"Oh, really?" He plays along, "Let's see then."
Amy's laughs fill the room as soon as he starts tickling her hips. She twists trying to counter attack, and eventually manages to slip her hand over his armpit to tickle back. Leonard laughs along.
"What the heck is going on here?
Leonard and Amy freeze hearing Sheldon's voice. Quickly they leave the other's personal space.
"We experimented with beans," Amy calmly explains, "And then I was proving Leonard a girl can easily win a tickle fight with him," she adds as she turns back to Leonard.
"Hey," Leonard protests, "I was totally winning!"
"Oh, you shut up!"
Leonard and Amy stare surprised at Sheldon. He has sounded pretty much annoyed, and almost mean. They are both genuinely confused by such a reaction, and clearly upset.
"Buddy," Leonard tries to say, "I swear there's nothing - Sheldon, wait!" He ends up calling as his friend just turns back and rush away.
Amy immediately stands and goes after him back in his room.
"Sheldon, what's wrong?" She asks once they are there.
Now that they are alone, he feels okay to speak. "Isn't it obvious? I am jealous."
"This is kind of flattering, but you know it also does not make any sense, right?" Amy checks.
"Of course," Sheldon immediately dismisses, "I don't mean in a romantic way."
Amy gets closer. Sheldon seems to be reassured by her proximity, and even attempts a shy smile.
"You spend so much time together. I envy that. I miss you."
"Sheldon…"
This totally caught her off guard. "I miss you, too."
Their gazes meet. He smiles, "Good."
"I also know that you need to be alone," Amy continues, "following this new, promising path in string theory, which maybe will finally bring you to some result. I know it's worth it. I know it's important."
"So are you."
She loves he said that with that tone. The one that means that it's a matter of fact, it's almost useless saying it again. She reaches his face and kisses him.
"It's nice to hear this," she whispers before stealing another kiss. Sheldon simply kisses her back, and catches her in a tight hug. They have already deepened the kiss, and he is considering to pushing her against the wall and just unbutton that pretty cardigan of hers…
"Oh no, no, sorry guys!" They when they hear Leonard, who has entered the room and now is covering his eyes, hoping to not see anyone with any missing piece of clothing.
They detach. Sheldon looks at him, and Leonard is glad to see he has a very different expression now. "How can we help you?" Sheldon asks.
Leonard still feels a little unsure, but goes for it anyway. "I was thinking that maybe for tonight we may do something the three of us together. The next project in the list is a classic - baking soda volcano."
"It works for me" Sheldon enthusiastically accepts. Amy nods along, and he takes her hand right away to follow Leonard back in the living room.