The Big Bang Theory Season 8 Episode 16
The Intimacy Acceleration
Amy tells the group about an experiment to make people fall in love with a few hours. For the sake of science, Sheldon and Penny decide to try the experiment themselves.
Meanwhile, Howard and Bernadette fly out to Howard's mother's funeral. Their baggage has been lost, which includes his mother's ashes.
Leonard and Amy tag along on a date with Raj and his girlfriend.
Penny and Sheldon start their experiment where they ask each other personal questions, including what their perfect day would be. Notably, each of them leave out their significant others in their descriptions.
The conversation between Penny and Sheldon becomes serious, and Penny realizes Sheldon is bothered that he has a hard time understanding people.
The rest of the gang participates in an interactive zombie game that turns out to be a bit too easy.
Howard has an emotional moment in the airport, sad that he didn't spend enough time with her. Luckily, the ashes are finally found.
Penny and Sheldon continue their experiment, which has them staring into each others eyes for four straight minutes. The experiment results in a new found intimacy, but not the kind of love the experiment was aiming for. Instead, they realize the closeness of their friendship. Penny also finally learns when Sheldon's birthday is, and it happens to be today.
She manages to leak that news in order to throw a surprise party for Sheldon in the end of the episode.