The Cushion Saturation

The Cushion Saturation is the sixteenth episode of the second season of the CBS sitcom, The Big Bang Theory. This episode first aired on March 2, 2009.

Summary
Penny accidentally shoots Sheldon’s sofa cushion with a paintball gun. He is deeply disturbed by this turn of events and is not happy with the cushion even once it has been dry cleaned. Meanwhile Howard starts sleeping with Leslie Winkle. She uses her control over Cal Tech grant money to make him do things that she wants.

Extended Plot
Leonard, Sheldon, Howard and Rajesh are playing paintball and are losing badly, which Sheldon believes is due to Howard missing his Physics Department Paintball Strategy Meeting. After everyone leaving Howard to die, Leslie Winkle shows up, and after discussing how they might die very soon, they kiss. Penny goes to Leonard for help on her computer, and after being shown his paintball gun shoots Sheldon's cushion, leaving it with a big green spot. Leonard tries, and fails, to clean it, and immediately blames Penny when Sheldon finds out. They get the cushion covering dry-cleaned, but Sheldon is still not happy with it, and refuses to sit in it. Leonard tries to take his mind off of it by confessing that Szechuan Palace has been closed for two years, and his Cashew Chicken comes from Golden Dragon in Szechuan Palace containers. In the end, Sheldon is still dissatisfied with the dry-cleaned cushion, and he team-kills Penny in the next paintball game, prompting the others to "surrender, then (go to) Denny's".

Meanwhile, Leslie and Howard start sleeping together, and she is able to secure funds for his work, and even invites him to a trip to the CERN Super Colliderproject in Geneva. However, this turns out to be just Leslie's way to control Howard. Although initially reluctant to be in this sort of relationship, Howard later happily accepts his role as a "sex toy/arm candy".

Quotes
Sheldon: That is my spot. In an ever-changing world it is a simple point of consistency. If my life were expressed as a function in a four-dimensional Cartesian coordinate system, that spot, at the moment I first sat on it, would be [0,0,0,0].

Trivia

 * Sheldon says, "If my life were expressed as a function on a four-dimensionalCartesian coordinate system, that spot at the moment I first sat on it would be $$(0,0,0,0).$$" The Minkowski spacetimecombines space and time in a single, four-dimensionalmanifold $$(\textrm {dim} \ \mathcal {M} = 4),$$ which is covered by coordinate charts. $$x^{\alpha}({\alpha} = 0, 1, 2, 3).$$ Coordinate charts are used in the spacetime manifold $$\mathcal {M}$$ to represent the events. Usually, Cartesian coordinates $$x^{\alpha} = (ct, x, y, z)$$ are used, though curvilinear coordinates are also allowed. In the most general case, a number of overlapping coordinate charts is needed to cover the entire manifold. A reference frame is identified with a Cartesian coordinate chartwith a physical observer placed at its origin.
 * There is a plot hole towards the end of the episode when Penny joins the paintball team with Leonard, Sheldon, Raj, Howard and Leslie. Sheldon shoots Penny, and Penny retaliatesbut Sheldon claims that, "She can't shoot me, she's dead!" followed by Leonard agreeing with him and shooting Sheldon. Sheldon then shootsand killsLeonard. Sheldon had already been killed by Penny and therefore could not have killed Leonard.

Critics
"If Leslie and Howard stay together for form a relationship then this episode will have served a purpose." - The TV Critic's Review