The Prestidigitation Approximation

The Prestidigitation Approximation is the eighteenth episode of the fourth season of the CBS sitcom, The Big Bang Theory. This episode first aired on March 10, 2011.

Summary
Priya feels threatened by Leonard hanging out with his ex-girlfriend Penny all the time. Sheldon attempts to figure out Howard's magic trick.

Extended Plot
The episode begins with Leonard and Priya in bed together. They discuss the racism implied in Leonard's attempt in trying out positions from the Kama Sutra, and shortly after, Priya urges him to get contact lenses as she claims he has beautiful eyes.

Later, at the Cafeteria, Howard offers to show Raj a magic trick; Sheldon intervenes and tells him he shouldn't be wasting his time with such trivial matters. Howard performs the trick anyway and impresses both Raj and Sheldon. Although Sheldon mocks the trick, he is still keen to unravel it. Leonard comes in with his new contact lenses which are clearly incompatible with him, and he crashes into several people and furniture.

In the next scene, the crew is back at the apartment, and Howard impresses Penny with his magic trick as Sheldon watches carefully, apparently still stumped by the card trick. Leonard comes back from shopping for more grown-up clothes with Priya, the two head for his room. Priya voices her feelings about Leonard hanging out with Penny all the time and asks him to terminate their relationship, and he reluctantly agrees. Meanwhile, Howard continually impresses his friends with the magic trick, and Penny appears to have successfully figured out the magic trick, which further crushes Sheldon's ego.

Leonard and Penny bump into each other in the laundry room, where she "saves" Leonard's new silk shirt that Priya bought for him, seeing that he was about to ruin it by throwing it into the washing machine. She then jokingly says, "What will you do without me?", which makes Leonard feel guilty about the promise he made to Priya, which he knows he must fullfill. He nervously tries using Darwin's finches and his theory of natural selection as an allegory for their current relationship, and tries to hint that he would like to see her less without saying it directly. His obscure method "cutting their ties" confuses Penny, and she leaves the room thinking no different of their friendship. Later, when Priya questions him about whether or not he spoke to Penny about this issue yet, he hastily confirms that he has, hoping that the issue is now settled.

Meanwhile, Sheldon has grown increasingly obsessive with understanding Howard's magic trick, and has even gone as far as hacking into the Oak Ridge National Laboratory's Cray supercomputer to analyze shuffling patterns and looks into procuring Uranium-235 off of Craigslist after wondering if Howard used a radioactive tracer.



In another scene, Priya and Leonard are having dinner in an Indian food restaurant, and Priya has been offered a job in Los Angeles. Leonard is thrilled at this news and urges her to relocate to Los Angeles. At this moment, Penny shows up in the window their table is next to, and she enthusiastically taps the glass of the window, waving to them. She enters the restaurant and strikes up a conversation with the two, and it becomes apparent to Priya that Penny is completely oblivious to the fact that Leonard was supposed to terminate their friendship.

At the Cafeteria, Sheldon claims he has solved and improved Howard's magic trick, but it turns out he has bar coded all the cards and used a reader to identify it. Howard feels that Sheldon's execution was pathetic and shows the trick again, but this time it is revealed to the audience that Raj (and Penny) has been playing along with Howard's wild guesses as a mean joke on Sheldon, knowing that he would go nuts trying to figure it out.

Later, Leonard finally musters up the courage to cut his ties with Penny, and it quickly dawns on her that there are complications in their friendship now due to their previous relationship and Leonard's current relationship with Priya, and she agrees to keep a distance (though it obviously doesn't last).

Sheldon is depressed that he is unable to solve the magic trick.The Department of Homeland Security busted him for hacking into the Cray supercomputer and for trying to buy Uranium-235 over the internet. (Rather than arresting him, the DHS let Sheldon's mother deal with him, which Sheldon claims is a far worse punishment.) Howard offers to show it again. However, this time Sheldon requests Howard try it on him, and Howard reluctantly agrees. Sheldon isn't "gullible" like Raj is, and with his finely tuned senses, he will spot Howard's trick as soon as Howard screws up. But with all of his focus on Howard, Sheldon doesn't notice Raj signalling to Howard that Sheldon's card is the two of hearts. Furiously, Sheldon storms out of the cafeteria as Raj and Howard dissolve into laughter. 

Trivia

 * Leonard tried to use contacts in the 7th grade.
 * Leonard was his school's debate club president.
 * Leonard has two Star Trek uniforms, everyday and dress. One appears to be a beige wraparound tunic and the other a green-gold shirt, both command division colors. In The White Asparagus Triangulation, Sheldon refers to Leonard as Kirk in his "landing party". Raj has a Lieutenant Uhura uniform, which Priya and her family are embarrassed to know. Leonard asks, "Hypothetically, if I had access to a Lieutenant Uhura uniform, would you wear it?" She finally wears it for Leonard in The Roommate Transmogrification.
 * Leonard can hear Penny's toilet flush from his apartment.
 * This is the first time we see the Tandoori Palace which has been referenced in previous episodes.
 * Sheldon's board displays the number of ways (52! or approx. 8 &times; 1067) to shuffle a deck of playing cards.
 * Raj's birthday is October 6th.
 * A poster for neutrinos is visible in the cafeteria.
 * This isn't the first time Sheldon has been busted by the government. In The Luminous Fish Effect, Sheldon's mom said that as a boy, Sheldon tried to build a nuclear reactor in the barn, but was foiled when two men from the government showed up and told him that his project was illegal.