The Big Bang Theory Season 1 Episode 16 : The Peanut Reaction

When Penny learns that Leonard has never had a birthday party, as his parents didn't believe in celebrating anything but achievements. She decides to throw him a surprise party, and blackmails Sheldon to join her. However, when the day of the party arrives, Penny finds Sheldon did not buy a birthday gift, so she has to drive him to a computer store to pick up a present.