Leonard is losing at Sheldon's hand in a game of 3-Dimensional Chess . Penny has just returned from Nebraska and stops by for her mail, she adds "It was the worst trip. Everyone got sick over the weekend."
Sheldon springs up and distances himself from Penny and starts spraying the area, as he is afraid he may catch a cold. Leonard quickly dismissed this action as another one of Sheldon's overreactions. Later that night, Sheldon asks Leonard to place a measuring cup in the bathroom so Sheldon can measure his urine output to make sure his kidneys isn't shutting down. Leonard quickly becomes angry because he mixes pancake batter in that cup. After Sheldon proves to Leonard that the measuring cup was always used to measure urine, Leonard sarcastically says he owes the Betty Crocker company a letter of apology. By the next morning, Sheldon discovers he is sick.
Leonard quickly mobilizes and dashes out of the apartment to avoid Sheldon. When Sheldon calls to find him, Leonard lies, telling Sheldon that he got called in to work (yes, at 6:30 in the morning, and yes, on a Sunday). When Sheldon hears a dog in the lobby barking in the background, Leonard makes up an excuse about dogs being trained in the lab to operate centrafuges. Leonard, Howard and Raj decide to go to a "Planet of the Apes" movie marathon screening to lay low and avoid Sheldon.
With no one to wait on him, Sheldon goes to the Cheesecake Factory for soup. Even with his 187 IQ, Penny has to point out that Sheldon could have gotten soup delivered, rather than venture out. Sheldon asks about split pea soup with hot dog slices and croutons like his mother used to make. Penny advises that they have potato-lentil soup or chicken tortilla soup. Sheldon says: "surprise me". As Penny goes to get his soup, he blows his nose into his napkin and turns to a family sitting at a table next to him, asking, "...would you call this moss green or forest green?"
After Penny drops Sheldon off at his apartment after leaving work early, out of sympathy she gets pulled into taking care of him. He tells her that when he was a visiting professor in Germany, he got sick. His mom flew back to Texas to help Sheldon's dad with their house. Their house was aluminum [i.e, a pre-fabricated or mobile home and had slid off the cinder blocks again. He told her that a maid gave him an enema.
Back at the screening, just as the ten and a half hour marathon started, Howard accidentally breaks Leonard's eyeglasses after he dropped them, rendering him sightless. Sheldon begs Penny to rub some Vicks VapoRub on his chest, he also asks her to sing Soft Kitty to him, a song his mother always sang to him when he was sick.
As Leonard doesn't want to watch the marathon blurred by his nearsightedness due to breaking his eyeglasses, the three go back to the apartment in an attempt to retrieve Leonard's spare glasses. Leonard is caught by Penny, at which point she calls him a "Rat Bastard". Howard and Raj abandon him. An angered Penny alerts Sheldon of Leonard's presence. Leonard, still blinded, makes a mad dash to escape, but runs into one of the apartment's pillars. Finally on the couch sits Leonard with an ice bag on his head and a sick Sheldon.
- Guest starring:
- Teleplay: Bill Prady & Stephen Engel
- Story: Chuck Lorre & Lee Aronsohn
- "A very dull episode. The humor is built around Sheldon being annoying and it just isn't convincing or funny." - The TV Critic's Review
- IMDb user reviews
- Title Reference: The title refers to the anomalous cross-contamination between Sheldon's urine cup and Leonard's pancake batter.
- Chuck Lorre's vanity card 
- This episode was watched by 8.68 million people with a rating of 3.1 (adults 18-49).
- Episode transcript .
- Sheldon revealed that Leonard is a poor chess player in the previous episode (S1E10), and it is shown how he is not well suited for three-dimensional chess in this episode. Penny can also beat Leonard at chess in "The Werewolf Transformation" (S5E18). Leonard would do well to read the Federation Standard Tri-D Chess Rules.
- The pseudoscalar meson nonet  is clearly seen on the small refrigerator blackboard, which is what Eric Gablehauser was "doodling" in "The Luminous Fish Effect" (S1E4).
- Code Milky Green is the emergency code for when Sheldon is sick, possibly because of the color of mucus build up.
- "Möchtest Du eine Darmspülung?" Translates into "Do you want an enema?". Although it should be noted, that while technically not wrong, the medically correct German term would be "Einlauf", not "Darmspülung".
- When Sheldon comes out looking for Leonard, he drops the comforter to the floor before he enters the kitchen. In the next shot as he approaches the refridgerator, watch how the comforter has crawled down the step on its own.
- The song "Soft Kitty" is introduced. Penny learns it and sings it to sick Sheldon.
- Sheldon states he has an IQ of 187, and in "The Codpiece Topology" (S2E2), he says he has an IQ which can’t be accurately measured by normal tests and he’d have to lose 60 IQ points to be classified as smart. If Sheldon's IQ is indeed 187, then Leonard's IQ would be 173, as Leonard said their combined IQ is 360 in "Pilot" (S1E1).
- Sheldon claims that this time is only the second time he has been sick by himself. The first was when he was 15, at the Heidelberg Institute in Germany as a visiting professor (This very institute does not exist. What is meant here is the renowned "Heidelberg University" with its many diverse institutions). The sausage based cuisine causes "an intestinal blitzkrieg", and his mother had to return to Texas to help deal with their aluminum house slipping off its cinder blocks during tornado season.
- Leonard advises Sheldon to "get some rest and drink plenty of fluids". With characteristic appreciation and charm, Sheldon responds, "What else would I drink? state of matter Gases? Solids? Ionized plasma?"
- Even under the stress of illness, Sheldon retained his rapier wit sufficiently to crack a "physics funny". It's "common knowledge" from Physics 101 class that gases and plasma are also fluids, just like liquids, and make refreshing "virgin" drinks.
- Technically, the applicability of a fluid approach for a collection of free particles (gases) lies in comparing the mean free path () to the macroscopic length scale () of interest. For ionized gases, the subject goes by the name magnetohydrodynamics Plasma (when the methods of continuum mechanics apply, and by plasma when they do not.
- The domed building visible from Leonard and Sheldon's apartment 4A window is the Pasadena City Hall.
- When Sheldon was trying to test if he was sick, he had to use lime Jello to make petri dishes because "SOMEONE polished off the apricot yogurt," implying it was Leonard. While Leonard is lactose intolerant, if his intolerance is mild or moderate, he could eat yogurt if it had live active cultures.
- This is the first time that Penny calls Leonard a "Rat Bastard." The second time she does it is in the Season 3 finale of "The Lunar Excitation" (S3E23).
- In this episode, Sheldon said at the age of 15, he got sick in Germany, and his mother had to fly back to Texas to help his dad at that time. This creates a continuity error in the episodes "The Thanksgiving Decoupling" (S7E9) and "The Proton Transmogrification" (S7E22) because Sheldon mentions to Bernadette's father Mike and the ghostly form of Professor Proton, respectively, that his father died when he was 14.
- On The TV version of the episode, the cold open ends which Penny closing the door and Sheldon using a disinfectant spray, but on the Season 1 DVD, An extended scene shows Leonard looking at him and Sheldon says "What?!"
- Before the show began, presumably, Sheldon had an ear infection in 2006 which is mentioned by Howard.
- (Howard is asleep, when the phone rings.)
- Mrs Wolowitz: (1st time: she's shouting from downstairs off-screen) HOWARD, IT'S THE PHONE!
- Howard: (he yells in giant fury) I KNOW IT'S THE PHONE, MA! I HEAR THE PHONE!
- Mrs Wolowitz: (2nd time: she's shouting from downstairs off-screen) WELL, WHO'S CALLING AT THIS UNGODLY HOUR?!
- Howard: I DON'T KNOW!
- Mrs Wolowitz: (3rd time: she's shouting from downstairs off-screen) WELL, ASK THEM WHY THEY'RE CALLING AT THIS UNGODLY HOUR!
- Howard: HOW CAN I ASK THEM WHEN I'M TALKING TO YOU?!
- Howard: [Answers the phone] Hello?
- Leonard: (in the apartment lobby, hiding from Sheldon.) Hey Howard, it's Leonard. Code: Milky Green.
- Howard: Dear Lord, not Milky Green.
- Leonard: Affirmative, with fever.
- Mrs. Wolowitz: (4th time: she's shouting from downstairs off-screen) WHO'S ON THE PHONE?!
- Howard: IT'S LEONARD!
- Mrs. Wolowitz: (5th time: she's shouting from downstairs off-screen) WHY IS HE CALLING?!
- Howard: SHELDON'S SICK!
- Mrs. Wolowitz: (6th time: she's shouting from downstairs off-screen) WERE YOU PLAYING WITH HIM?!
- Howard: FOR GOD'S SAKE, MA, I'M 26 YEARS OLD!
- Mrs. Wolowitz: (7th time: she's shouting from downstairs off-screen) EXCUSE ME, MR. GROWN-UP! WHAT DO YOU WANT FOR BREAKFAST?!
- Howard: CHOCOLATE MILK AND EGGOS, PLEASE!
- Leonard: Howard, listen....
- [Phone beeps.]
- Howard: Hang on, call waiting.
- Leonard: Wait, NO, don't -
- [Howard takes the call.]
- Howard: Hello?
- Sheldon: Howard, I'm sick!
- (Howard doesn't know what to do until he decides to pretend to be his mother answering.)
- Howard (as Mrs. Wolowitz): Uh, HOWARD'S SLEEPING, THIS IS HIS MOTHER! WHY ARE YOU CALLING AT THIS UNGODLY HOUR?!
- Sheldon: I need soup!
- Howard (as Mrs. Wolowitz): THEN CALL YOUR OWN MOTHER! (To Leonard) It was Sheldon.
- Leonard: I tried to stop you.
- Howard: It’s my own fault, I forgot the protocol we put in place after the great ear infection of ’06.
- Leonard: You call Koothrappali, we need to find a place to lay low for the next eighteen to twenty four hours.
- Howard: Ma, can my friends come over?
- Mrs. Wolowitz: (8th time: she's shouting from downstairs off-screen) I JUST HAD THE CARPETS STEAMED!
- Leonard: Five movies, two hours apiece. It’s a start.
- [Raj steals some of Howard's popcorn.]
- Howard: Take your stinking paws off my popcorn, you damn dirty ape.
- Sheldon: Can you sing “Soft Kitty”.
- Penny: What?
- Sheldon: My mom used to sing it to me when I was sick.
- Penny: I’m sorry, honey, I don’t know it.
- Sheldon: I’ll teach you. “Soft kitty, warm kitty, little ball of fur, happy kitty, sleepy kitty, purr purr purr.” Now you.
- Penny: (after a loud sigh) Soft kitty, warm kitty…
- Sheldon: Little ball of fur. Keep rubbing.
- Penny: (through gritted teeth) Little ball of fur.