The gang is having Thai food at Leonard and Sheldon's apartment. Howard suggests to Bernadette that they should leave for the movie they want to see; however, Raj, thinking that he is invited, gets up to go too and, much to Howard's dismay, he is invited along. Penny has to leave for work as well, following which Sheldon asks if someone can drive him to the new model train shop in Monrovia. Soon enough, everyone has left except for Amy and Leonard who, despite Leonard's discomfort, stays and reads a book. She asks if she should go, as she has been told that she overstays her welcome sometimes. Leonard, although feeling a little awkward, tells her that she can stay as long as she likes.
Meanwhile, Raj, Sheldon, Bernadette, and Howard are en route to their destinations. Sheldon is going to the model train store, as they are having a lecture about HO gauge railroading, something of which he heartily disapproves. Howard is incredulous as to how someone as old as Sheldon still plays with toy trains, but Bernadette points out his hypocrisy, as Howard has a closet full of magic tricks. Howard defends this hobby by referring to them as "illusions", and says that they will be going in the attic once they're married so that she can use the closet for her clothes. Bernadette asks why she would be keeping clothes at his mother's house. Howard says to think of it as their house, but that his mother wouldn't be moving out. Bernadette becomes upset that Howard believes that she will live with him and his mother and tells Raj to take her home, but Howard won't allow it, saying to still go to the movies. Sheldon tries to settle the argument by suggesting to Raj to take him to the train store first, adding that he doesn't care what Raj does afterwards.
Back at the apartment, Leonard has just come out of the shower and Amy is still there, reading his high school yearbook and concludes that he was not at all popular back then. She softens the blow by adding that only her mother signed her yearbook with a remark about her hymen. At that moment, Sheldon comes home from the train store and says that he will never go back there again, due to being convinced to buy an HO gauge model train set. Amy finally leaves, but not before she asks Leonard if he can come with her to a wedding that Friday. Leonard agrees, but asks why she won't take Sheldon. She says it's because the last time he came to a wedding with her, he behaved like a child the whole time there. Sheldon defends this by saying that she promised that there would be scientists his own age.
Later that night, Bernadette goes over to the Wolowitz house and apologizes to Howard about their fight in the car. Howard suggests that she have a trial run with staying at their place. Howard asks his mother if it's OK, which she says is a relief after all the sleepovers with Raj. When she's there at the weekend, Howard asks if he and Bernadette can have some privacy, which Mrs. Wolowitz sees as them wanting to have sex. Bernadette gets a little uncomfortable, but Howard assures her it will be fine, until his mother calls out to let her know when they're done so she can have a foot rub.
At the wedding, Amy is having a wonderful time, but Leonard is feeling a little upset, as the wedding reminds him of his "kinda-sorta" girlfriend, Priya. Amy tells him that she has a "kinda-sorta" boyfriend, Sheldon, back home who is playing with toy trains, but that she isn't "bitching" about it. She then suggests that they go out on the dance floor, but Leonard says he isn't much of a dancer. Amy says she will lead, and they go and do the Chicken Dance.
Back at the Wolowitzs' house, Bernadette needs to brush her teeth, but Mrs. Wolowitz has been in the bathroom with constipation for almost an hour. Howard asks his mother to give up so Bernadette can use it. But Mrs. Wolowitz says she can come in, as she won't be embarrassed. Howard says to Bernadette that the problem is solved. All of a sudden, Mrs. Wolowitz finally has success, freeing up the bathroom; however, all this time Bernadette is very disgusted.
After the wedding, Amy is helping Leonard up the apartment stairs after he pulled his groin while dancing the Hokey Pokey. Leonard thanks Amy for a great time as well as breaking the head off the ice swan so he could put it against his groin. She is very good as spatial reasoning. Leonard then kisses Amy on the cheek as a friendly gesture, which causes her to rush over to Penny's place, fearing that she seduced him. She tells Penny that she will e-mail Leonard in the morning about how he can never have her body, adding that Penny has a better shot with her than he does, which doesn't thrill Penny.
After Sheldon tells Leonard that he has a great train night and asks him how his night was, Leonard says he had a great night with Amy, even though his groin hurts as a result. Sheldon then thinks that he had sex with Amy, so he karate chops Leonard and angrily tells him "She is not for you! Not for you!" Sheldon leaves, with Leonard looking completely confused.
Meanwhile, at the Wolowitzs' house, Bernadette comes in and says good morning to Howard. She says to him that she and his mother made him a breakfast of pancakes. Howard asks if she is getting along with his mother. Bernadette says its fine, but their communication is a little tricky as they are different people. When Mrs. Wolowitz asks Bernadette if Howard liked the pancakes, she screams out in a voice strikingly similar to Mrs. Wolowitz's that he hasn't tried them yet. Howard asks if there's butter with his pancakes and Bernadette tells him there is butter-flavored syrup. Mrs. Wolowitz calls out again to see what Howard thinks of the pancakes; Bernadette screams out in Mrs. Wolowitz's voice that he wants butter and so Mrs. Wolowitz calls out that it's butter-flavored syrup, but Bernadette cries out that she just told him that. Howard says that he doesn't need butter, but Bernadette calmly says to him in an American-Jewish accent, "If you want butter, I'll get ya butter." After Bernadette leaves Howard complains that he has to cut the pancakes himself, never noticing his mother's traits in Bernadette.
- Guest starring:
- Teleplay: Chuck Lorre, Eric Kaplan & Jim Reynolds
- Story: Bill Prady, Steven Molaro & Dave Goetsch
- Sheldon wears his "Riddler: Bat Question" t-shirt, as well as his checkered pajama robe.
- The TV Critic: "This is one of those times when I feel especially nuanced in my writing. I personally wouldn't build two plots around Howard and Amy's issues. But if you were going to then these weren't bad attempts."
- The A.V. Club gave this episode a B+
- IMDB user reviews
- Title Reference: Sheldon extrapolates (concludes) that Leonard and Amy had sex from Leonard's comment about his pulled groin.
- Chuck Lorre's vanity card.
- This episode was watched by 14.74 million people with a rating of 4.9 (adults 18-49).
- This episode aired in Canada on September 29, 2011 with 3.693 million viewers and a weekly ranking of #1.
- In the United Kingdom, this episode aired on November 10, 2011 with 1.26 million total viewers and a weekly ranking of #9.
- In Australia, it aired on October 17, 2011 with 1.488 million viewers and a weekly ranking of #4.
- Episode transcript 
- Second time the 4th wall in Leonard and Sheldon's apartment can be seen; the first was in "The Euclid Alternative" (S2E6).
- Amy didn't go to her prom. Her mother paid her cousin to go with her, but he spent the money on drugs. She did help clean up and danced with her mop per "The Prom Equivalency" (S8E8).
- Sheldon implies that Penny's mother is fat.
- Sheldon believes he is the most valued, important member of the group and leader.
- Apparently, when Howard says "Honey", it used to mean Raj.
- Sheldon creates a guessing game in this episode, where he ask the question: "This four letter word describe either, a printer's type size, OR compulsion to eat dirt." Although it is not mentioned, the answer is "pica".
- Mrs. Wolowitz says that after all of Howard's sleepovers with "the little brown boy", a girl staying over is a big relief.
- Until his HO gauge starter kit, Sheldon was all for "O gauge or no gauge". He then flirts with N-gauge, which is even smaller. And enthuses about the miniaturization involved, even putting one in his mouth.
- Sheldon's interest in trains has also been established in "The Bad Fish Paradigm" (S2E1), "The Terminator Decoupling" (S2E17), "The Irish Pub Formulation" (S4E6) and "The Zarnecki Incursion" (S4E19).
- His mother always cuts his meat for him when it's fatty.
- Amy draws a mustache on her finger for humor and as an icebreaker for conversations.
- Like in season 4's "The Engagement Reaction" (S4E23), Bernadette sounds like Mrs. Wolowitz when she screams and, once again, Howard doesn't notice, only complaining that there is no butter and that he has to cut the pancakes himself.
- Conclusion of the axion calculation saga on the whiteboards that began in "The Skank Reflex Analysis" (S5E1) and explored if a dense hot plasma could make the elusive axion particle. Based on the work of Dr. David Saltzberg, the laboratory plasma generated by imploding a small pellet with an array of 192 lasers at the National Ignition Facility would be incomparable to the solar plasma in the core. The net reaction used (deuterium + tritium -> helium + neutron + energy) occurs 25 orders of magnitude faster than the proton-proton fusion reactions in the Sun that produce the charged particles thought to radiate axions efficiently. An unhappy face was included on one of the whiteboards, consequently.
- First obvious sign regarding Sheldon's possible romantic feelings for Amy shown. When he asked Leonard how his night at the wedding was and Leonard explained that he never thought that he would have that much fun with Amy and added that she really knew how to help loosen up a guy and that his groin was a little worse for wear, Sheldon karate-chops Leonard and says "She is not for you! Not for you!" Sheldon thinks that they may have had sex.
- Johnny Galecki and Mayim Bialik also went on a date together on Mayim's series Blossom in 1991.
- The train set Sheldon bought is the Rail King made by Bachmann. It is no longer available, but a very similar set called Rail Chief is in production.
- Leonard decides that he likes Amy, even though he put up with the same behavior pattern from Sheldon for years.
- A Department of Primatology exists at Caltech (in-show). Amy needs to get her experimental monkeys and apes from somewhere, after all. Doctor Morelli and Doctor Gustafson are apparently the resident Brangelina couple; Amy gets an invite to their wedding.
- Sheldon is wrong that Amy and Leonard engaged in coitus. Sheldon was Amy's first lover in "The Opening Night Excitation" in 2015.
- Amy: Leonard, you may not have noticed, but I am being a delight here. And you're not holding up your end of the evening.
- Leonard: I'm sorry. The wedding just reminds me of my kind-of-sort-of girlfriend 9,000 miles away.
- Amy: I have a kind-of sort-of boyfriend who's playing with a model train right now, and you don't hear me bitching about it.
- Leonard: (sarcastically) You sound like you had a great night.
- Sheldon: I did! How was yours?
- Leonard: Not bad. I had a lot more fun with Amy than I thought I would.
- Sheldon: What exactly do you mean by that?
- Leonard: Well, turns out she really knows how to help a guy loosen up and have a good time. Although, truth be told, my groin is a little worse for wear.
- (Sheldon hits Leonard in the shoulder)
- Leonard: Ow! Why did you do that?
- Sheldon: To send you a message. She is not for you!
- Leonard: What?
- Sheldon: NOT FOR YOU!
- Sheldon: All these years, I've been so wrong. The tinier the train, the more concentrated the fun!
- Penny: (about Leonard) So handsome! Like James Bond.
- Sheldon: Better than James Bond, because he's tinier.
- Sheldon: It's official! I'm an HO trainiac!
- Amy: How was your shower?
- Leonard: It was good, good. Just out of curiosity, what time do you usually go to bed?
- Amy: Oh, I’m up all night. I’m like a possum. Boy, you were not liked in high school, were you?
- Leonard: Not really. Is that my yearbook?
- Amy: Mm-hmm. Dear Leonard, you’re really good at science. Maybe one day you’ll come up with a cure for being a dork.
- Leonard: Well, it wasn't spray-painting a lightning bolt on my briefcase, I can tell you that.
- Amy: If it makes you feel any better, the only person who signed my yearbook was my mother.
- Leonard: Aw.
- Amy: Dear Amy, self-respect and a hymen are better than friends and fun. Love, Mom.
- (The ending bedroom scene in the Wolowitz house where Howard is asleep in a not-so-tidy style when Bernadette enters with a breakfast tray of pancakes on a plate, a jug of butter-flavored syrup, a glass of orange juice and a mug of coffee and she suddenly arrives at Howard's bed)
- Bernadette: Good morning, handsome.
- (Howard struggles to wake himself up).
- Howard: (he groans in his sleep) Morning, Mom.
- (Bernadette is now very cross when she is being referred to as 'mom' by Howard).
- Bernadette: (she corrects him crossly) It’s me.
- Howard: (he is recovered) Yes, it is, and you’re so pretty in the morning.
- Bernadette: Your mom and I made you breakfast.
- Howard: Oh, wow. So you guys are getting along?
- Bernadette: (she's explaining sadly to Howard) Yeah, I guess. We’re very different people, Howard, so communication’s a little tricky.
- Mrs. Wolowitz (off): Does he like the pancakes?!
- (Bernadette yells with huge loud anger at Howard's mom's voice in the background)
- Bernadette (1st time: she shouts in Mrs. Wolowitz's voice): He didn’t try them yet!
- Howard: (asking shiftily) Is there any butter?
- Bernadette: (with a firm smile) It’s butter-flavored syrup.
- Howard: Oh.
- Mrs. Wolowitz (off): So, what’s the word?
- (Bernadette yells with huge loud anger at Howard's mom's voice in the background again)
- Bernadette: (2nd time: she shouts in Mrs. Wolowitz's voice) He wants butter!
- Mrs. Wolowitz (off): It’s butter-flavoured syrup
- (Bernadette indicates Howard and the breakfast tray to Howard's mom's voice in the background and yells with huge loud anger at her for the final time)
- Bernadette: (final time: she shouts in Mrs. Wolowitz's voice) I just told him that!
- Howard: (chuckling shiftily) I don’t need any butter.
- Bernadette: (she's so tired and really furious) If you want butter, I’ll get you butter.
- (Bernadette now exits the bedroom in fury. Howard now gazes around the bedroom and at the breakfast tray at the same time fore eight seconds and he now complains to himself)
- Howard: (he grumbles for a second) Well, I guess I’ll cut these by myself.
- ↑ The TV Critic's Review
- ↑ The Big Bang Theory: “The Pulled Groin Extrapolation” - TV Club - The A.V. Club
- ↑ Thursday Final Ratings: 'X Factor,' 'The Big Bang Theory,' 'Grey's Anatomy,' 'The Office,' 'The Secret Circle' & 'Mentalist' Adjusted Up - Zap2it
- ↑ Top Programs – Total Canada (English) September 26 – October 2, 2011 - BBM Canada
- ↑ BARB via wikipedia
- ↑ Free To Air TV Ratings, Week 43, October 16 – October 22nd, 2011 - Throng.com.au