The Cooper-Hofstadter Polarization
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.
The Grasshopper Experiment
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.
The Dumpling Paradox
With Wolowitz AWOL with one of her friends, Penny's recruited to fill in for him on the guys' Halo team
The Middle-Earth Paradigm
Penny hosts a Halloween party, putting Leonard, Sheldon, Wolowitz, and Koothrappali through a new test of their social skills.
The Tangerine Factor
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.
The Peanut Reaction
Penny wants to throw a surprise party for Leonard, but Sheldon's indulgence at an electronics store throws her off-track.
The Hamburger Postulate
Fearing he's wasting his time on Penny, Leonard ends up making a connection with Leslie.