Season 1, Episode 3 - The Fuzzy Boots Corollary
Aired October 8, 2007
Leonard's hopes of a relationship with Penny are dashed when he sees her kiss another guy, so he decides to ask his colleague Leslie Winkle out. When Leslie is uninterested in going out with Leonard, he feels dejected and thinks about getting a cat. In an attempt to buoy Leonard's spirits, and stop him getting a cat, Sheldon encourages Leonard to ask Penny out on a date.