The Toast Derivation
The Big Bang Theory Season 4 Episode 17 (02/24/11)
After Amy suggests to Sheldon that Leonard is the nucleus of the group, Sheldon decides to try and have an evening without his usual friends and invite some other people over, including Kripke, Stuart, Penny's ex-boyfriend Zack and Levar Burton. When Levar Burton doesn't show, Sheldon tries to steer the evening in the direction of his choosing, but the others resist and decide on their own fun.
Leonard has dinner at Raj's apartment with Priya, Raj and Howard. At first they all relish in the absence of Sheldon, but soon lament his whereabouts. Sheldon eventually caves in and comes to spend the evening with his friends.
Amy and Bernadette try to get Penny to go out for a night on the town to get her mind off of Leonard. Penny insists it doesn't bother her, but once she's reminded of the snowflake he gave her, she decides to go look for a one-night stand.