When Leonard and Sheldon encounter a young woman crying on the curb, Leonard invites her to dinner. She is abrasive, sexy, hard, and sarcastic, but she softens a little when she discovers that the guys aren't trying to get something from her.

Brilliant physicist roommates Leonard and Sheldon meet their new neighbor Penny, who begins showing them that as much as they know about science, they know little about actual living.

Leonard volunteers to sign for a package in an attempt to make a good impression on Penny, but when he enlists Sheldon for help, his attempt at chivalry goes terribly awry.

Leonard asks a woman out after he finds out that Penny is seeing someone.

Sheldon's getting fired forces him to explore what life has to offer outside physics, leaving Leonard to take drastic action to snap his friend out of his funk.

Fearing he's wasting his time on Penny, Leonard ends up making a connection with Leslie.

Penny hosts a Halloween party, putting Leonard, Sheldon, Wolowitz, and Koothrappali through a new test of their social skills.

With Wolowitz AWOL with one of her friends, Penny's recruited to fill in for him on the guys' Halo team

Raj's parents set him up on a blind date, but just when he discovers he can talk to her, she shows interest in Sheldon.

When they're invited to present at a physics conference, Sheldon's reluctance to share the credit leads to a tiff between him and Leonard, and Penny's intervention only exacerbates the situation.

Leonard and Sheldon each lie to avoid seeing Penny's concert, but Sheldon's is a bit too complicated for his own good.

With his friends knowing how sour he acts when sick—and avoiding him as a result—Sheldon is nursed by Penny, who hasn't a clue what she's in for.

With Sheldon rattled over meeting Dennis, a 15-year-old physicist who's smarter than he, Leonard helps him look for a way to debunk Dennis's research.

Sheldon finds himself booted from the gang's Physics Bowl team and replaced with Leslie, so he forms his own team, hoping to prove he's superior.

When delivery of another collectible leads to Penny's lashing out at Leonard, he decides it's time to give up his habit, much to his friends' protestation.

The rest of the gang fall for Sheldon's twin sister, and Leonard's plan to win her heart is derailed when Sheldon declares he'll be the one to decide who can date her.

Penny wants to throw a surprise party for Leonard, but Sheldon's indulgence at an electronics store throws her off-track.

Penny's break-up spurs Leonard to summon the courage to ask her out, and each goes to Sheldon for advice on their upcoming date.

When Sheldon learns the guys tampered with his expedition data he got from the arctic, he leaves to Texas in disgrace. This results in the guys following him, which threatens Leonard's hope for some romantic time with Penny and the guys' friendship with Sheldon.

Leonard and Penny struggle to recover from an awkward first hookup while Sheldon and Howard stake their best comic books on a bet to determine the species of a cricket.

Howard and Raj visit a Goth nightclub to pick up women while Sheldon attempts to build a better Penny using chocolate-based behavior modification.

Raj must find a new job or be sent back to India, but Sheldon has a solution, which leaves Leonard and Penny dealing with Howard as a third wheel.

After overhearing that Wil Wheaton is entering a trading-card competition, Sheldon decides to enter it himself. Meanwhile, Penny sets up one of her friends on a date with Wolowitz.

Sheldon becomes Leonard's teacher in order to understand football, while an incident with flying a kite threatens Howard and Raj's friendship.

A squabble between Leonard and Penny sees Sheldon playing mediator, though out of his own incovenience from the sitiation rather than altruism.

With Leonard, Howard and Raj away camping in the desert, an injured Penny has only Sheldon to rely on.

After he's humiliated on National Public Radio, Sheldon vows to destroy Kripke, while Wolowitz tries not to destroy his new relationship with Bernadette.

Penny gets help from Sheldon in order to understand what Leonard's work is, while Wolowitz develops a jealousy when Leonard starts hanging out with his new girlfriend.

A Christmas visit from his mother has Leonard discovering she's kept a lot of news from him that she shares with Sheldon and worrying if she'll approve of his romance with Penny.

Leonard is aghast to find out a secret of Penny's: she believes in psychics. Meanwhile, Sheldon tries to assist Koothrappali in picking up a girl at a university mixer.

When their apartment is robbed, Leonard and Sheldon turn to their friends to create a state-of-the-art security system.

Sheldon's search for the answer to a physics problem leads him to work at the Cheesecake Factory with Penny.

Leonard can only bring one guest on a trip to see CERN's Large Hadron Collider (the world's largest and highest energy particle accelerator) in Switzerland on Valentine's Day.

Sheldon winds up in traffic court when he gets a ticket because of Penny.

When the guys find a ring from "The Lord of the Rings" at a garage sale, it threatens to tear them apart.

Sheldon's friends come to his aid when his fear of public speaking stands between him and a coveted award.

A fight between Leonard and Penny threatens their relationship, while Sheldon battles Wil Wheaton in bowling.

When Leonard and Penny aren't speaking, Sheldon goes to extremes to keep them both happy.

Leonard and Sheldon compete for the attention of a famous female physicist.

Leonard tells Penny about how he met Sheldon for the first time...and what happened to the elevator.

While Penny worries that dating Leonard has ruined her for normal guys, Wolowitz and Koothrappali search for Sheldon's perfect match online.

Celebrating the 100th episode of US sitcom The Big Bang Theory, Rick Edwards travels to the Hollywood set for an access-all-areas visit, catching up with the cast and learning everything there is to know about the show.

This behind-the-scenes special takes fans through everything from the writing of the script to the recording of the show in front of a live audience.

Celebrating the 100th episode of The Big Bang Theory, Rick Edwards looks back at some of the US sitcom's most hilarious and memorable scenes and talks with the cast and crew on the set in Hollywood.

The actors share their favourite moments and reveal a few secrets about the making of the show.