Knock-knock-knock.

"Penny."

Knock-knock-knock.

"Penny."

Knock-knock-knock.

"Penny."

As ever, Penny humoured me and refrained from opening her apartment door until I had finished my compulsory knock-knock-knock routine. However, when it opened to reveal a visibly upset Penny, I was perturbed. "Is now a bad time? Should I come back later?" I enquired purely out of politeness, as social protocol dictated I should, but knew that I would be even more disconcerted if she said 'yes'. I had especially altered my routine to accommodate this unforeseen dilemma, and would not have been particularly happy if all the extra effort had been in vain.

"No, it's fine Sheldon, come on in." Penny sighed, stepping back to allow me entry to her apartment. "I've just split up with Leonard. Again. There's not going to be a 'good' time to disturb me for a while."

I stepped inside and she closed the door behind me quickly, as if she feared that Leonard would exit our apartment at the exact same moment that I entered hers, and that he would get the wrong impression. "I was only asking to be polite. I would have come in anyway, regardless of your answer."

Penny rolled her eyes and collapsed onto the sofa. "What's troubling you, Sheldon?" She made a feeble attempt to scrutinize my face, looking for clues. I was not so naive as to make my romantic feelings plain, after I had spent so long denying that I could feel them at all. "This isn't because you told me what Leonard was planning to do, is it? Has he been giving you a hard time? He has, hasn't he?" Penny sounded incredulous, which I thought was a little presumptuous as I hadn't even confirmed her suspicions. "That man has no right to take it out on you! What's happened is his fault, not yours."

I belatedly noticed that Penny was acting in my favour, choosing to defend me over the man with whom she had shared coitus. My newfound feelings must have been slowing my cognitive processing, or I would have realised this earlier. After all, Penny was not exceptionally intelligent, whereas I was. I had remarked once that this cognitive impairment had affected Leonard during one of his short-lived romantic relationships with a woman whom he had shared coitus on a regular basis. I had never shared coitus with anybody - anybody following me on Twitter would have been aware of this, alongside my friends and colleagues - but maybe the principle was the same, that relationships caused people to lose a little bit of themselves, whether it be their intelligence or their independence. In my case, I would be a piece of my dignity, after asserting for many years that I had no urge to have a romantic relationship with anyone or anything.

Before I could become lost in my own hypothesizing - I was currently torn between two theories; the first being that Penny's defensive stance in my favour was a sure sign of affection, and the second being that she was simply extremely annoyed at Leonard, with the stage in her current menstruation cycle not helping - I shook my head to dismiss my thoughts. Both situations were plausible, and therefore it was not helpful to dwell on them because without more evidence, the true explanation would remain elusive. "No. For once, my troubles are not of Leonard's design." Penny raised an eyebrow. Maybe my syntax had been a little confusing for her. I reluctantly rephrased to aid her understanding. "It's not Leonard who's bothering me."

Penny raised an eyebrow. "Who is it then?"

"You."

She rolled her eyes and moved to reopen the door. "Get out. I'm having enough of a hard time right now without listening to you explain how I've somehow upset you. What did I do, leave a hair on your bed when I sat on it the other day, or something equally as trivial as that?"

I didn't move. "I'm sorry, I shall rephrase again. You are bothering me because there is something about you that I don't understand."

"Are you saying that the great Sheldon Cooper has finally found something that he doesn't understand? That I, a community college drop-out, has somehow managed to baffle a man with an IQ of 187?" drawled Penny.

"Sarcasm?"

"Just a little bit."

"What about that time? Was that sarcasm?"

"It's irrelevant, Sheldon! What have I supposedly done to you, then? I'm waiting with bated breath."

I eyed her warily, still not convinced that she was telling me the truth, but decided to move on and deal with the more important - no, more alarming - matters so that I could leave and continue with my laundry. "Thoughts of you are occupying my mind at an increasingly alarming rate."

"Excuse me?" Penny looked bemused.

"You're excused. If I don't understand why this is happening, then it's hardly fair of me to expect you to."

"I didn't mean 'excuse me' like that, as if I've done something wrong that I need to be forgiven for. I meant it in more of a 'has-somebody-slipped-something-into-Sheldon's-milk-again' way."

"Oh, I wish that it was that simple, Penny."

"What are you talking about, Sheldon? Either start making sense, or get out. I'm tired."

"You know very well what I'm talking about. It can't have slipped your notice that I tolerate more of your irritating behaviours than I do with Leonard, Howard or Raj. On more than one occasion, I've abandoned what I was doing in order to come to your aid. However, I realise that these aren't very scientific observations, and so I've taken the liberty of performing an experiment on myself to prove my point."

It was a few seconds before Penny could muster herself to speak. This revelation was evidently coming as a shock to both of us. "What kind of experiment?" Penny eyed me wearily.

"Well, I've noticed that every time I think of you, my blood pressure and heart rate become elevated. Therefore, if you would agree to monitor these measurements over the next few minutes, I will be able to tell you if you are the cause of these increased rates, or if it is simply correlational." I produced a sphygmomanometer for Penny to use. I could measure my heart rate myself, allowing for the possibility that erroneous measurements can occur when participants in a study attempt to self-measure.

"What happens if it's correlational?"

"Then further research will definitely be required, because these symptoms could be indicative of any number of illnesses, some serious and some not, but at the minute, I believe the most logical cause to be a new form of attachment or bond that I have inadvertently formed with you."

It was blatantly obvious that Penny didn't believe a word I was saying. "Okay, sweetie. If I go along with whatever this is, will you leave me alone for a little while? I just need some space Sheldon, because this situation with Leonard has really hurt me. Again."

"Agreed." I held out the sphygmomanometer for Penny to record my blood pressure, and I set about recording my own heart rate. I was intelligent enough to allow for the minimal error that would have been caused by measuring it in this way. After we had both finished recording our readings, I needed to see them in order to compare them with those that I would expect to see when I was not under the influence of Penny, or any other extraneous variables. "Show me."

Penny complied, and I was shocked to realise that the findings did indeed indicate cause and effect, not just a correlation. Penny was causing my heart rate to accelerate, and blood pressure to rise. I suppose that I should have been mildly reassured and comforted that this meant that I did not have any serious illness, which would present with these symptoms. The reality of the situation however, was much more daunting. I was attracted to Penny. And she didn't even know it. All the physical symptoms were there. I would have asked Penny to confirm that my pupils had dilated, but I couldn't really rely on her answer being accurate, especially while she was in this particular emotional state.

"Sheldon, are you okay?" Penny was genuinely concerned for me, but I was more preoccupied in ascertaining that she hadn't really comprehended what I had said earlier. "What am I, a correlation or a cause?"

"Erm, correlation. I have to go." And with that lie I rushed out of Penny's apartment before the nervous ticks could start, startled by the revelation that I was attracted to Penny and she still blatantly believed it to be one of my quirks.

I was so preoccupied by my latest discovery, and the fact that I had proven it scientifically, that there were two things at the time that I failed to notice. The first was that Leonard was coming down the staircase from the floor above both Penny's apartment and ours, and consequently saw me leaving in a fluster that had been caused by the woman he purported to love. The second was that, in my haste to escape the truth, I missed Penny's final comment on our predicament as she looked in the mirror.

"I find it quite ironic that we were both so preoccupied by his heart rate and blood reassure that I've only just noticed that mine must have risen simultaneously when he entered the room in order for me to feel this restless. Oh great, my pupils are dilated and my face is flushing, too. Where did all this come from all of a sudden? I've only been single for twenty four hours!"

Author's Note: Sorry for changing the title, and any confusion it may have caused. I'm happy with the title now, so I won't be changing it again!