What exactly does that expression mean, "friends with benefits?" Does he provide her with health insurance?

You're probably thinking, "the comic book store, on a Thursday? Why I've fallen down the rabbit hole and into a land of madness." What you have failed to take into account, Penny, is that this is "Anything Can Happen Thursday"

Sheldon: More wrong? Wrong is an absolute state and not subject to degradation
Stuart: It's a little wrong to say to say a tomato is a vegetable, it's very wrong to say it's a suspension bridge

Leonard: I just don't get what she sees in Stuart, we're practically the same guy
Sheldon: ...Stuart is taller, artistic, self employed, and most importantly, gets 45% off comic books

Leonard: But we all agreed the third Thursday of every month will be "Anything Can Happen Thursday"
Sheldon: Apparently the news didn't reach my digestive system, which when startled has it's own version of "Anything Can Happen Thursday"

Listen to that! "Stomp, stomp, stomp." It's Wolowitz and his stacked heels that fool no one.

Sheldon [looking for musical instrument in new neighbor's boxes]: Do those sound like castanets to you?
Leonard: The box says "kitchen."
Sheldon: So? Do cocaine smugglers write "cocaine" on the box?

Sheldon: You'll effectively be paying yor self 5.19 per day
Penny: A day??
Sheldon: There are children in sneaker factories in Indonesia that outearn you

Sheldon: I'm a physicist. I have a working knowledge of the entire universe and everything it contains
Penny: Who's Radiohead?
Sheldon [after twitching for a minute]: I have a working knowledge of the important things

Sheldon: You won the Nobel prize, what, three years ago? So you must deal with a whole lot of "what has Smoot done lately?" My thought is we continue my research as team, you know, Cooper-Smoot, alphabetical.. When we win the Nobel Prize, you'll be back on top
Dr. Smoot: With all due respect, Dr Cooper, are you on crack?
Sheldon: Fine, Smoot-Cooper, what a diva

Leonard: Well, it seems once again you're caught between a rock and a crazy place.
Sheldon: Oh, I hate when that happens.

Leonard: You know what baffles me Sheldon?
Sheldon: Based on your academic record, I could guess any number of things

TBBT Quotes

Penny: Hey, Sheldon, did you change your Wi-Fi password again?
Sheldon: Yes, it's "Penny, get your own Wi-Fi." No spaces.

Sheldon: Why do you have the Chinese character for "soup" tattooed on your right buttock?
Penny: It's not "soup," it's "courage."
Sheldon: No it isn't. But I suppose it does take courage to demonstrate that kind of commitment to soup.
Penny: How'd you see it? You said you wouldn't look.
Sheldon: Sorry. As I told you, the hero always peeks.