The Nerdvana Annihilation
The Big Bang Theory Season 1 Episode 14 (04/28/08)
Leonard buys a replica time machine online from the 1960 film The Time Machine and ends up splitting it with the guys. As the boys move the giant machine into the apartment, they block the walkway and make Penny late for work. Penny lashes out at Leonard for his stupid nerd toys.
Leonard then goes home to get rid of all his "nerdmabellia," despite the guys trying to talk him out of it (or at least buy it off him!) Sheldon then confronts Penny about all her little kid things (Hello Kitty, Beanie Babies, etc) and she apologizes to Leonard. However, the thing that makes Leonard truly revert back is when he meets her new boyfriend Mike.