The nature of love is central to "The Intimacy Acceleration," the "The Big Bang Theory" Season 8, Episode 16.
The revolved around Amy's story of a test she has heard of. According to her, the test suggests that it it not impossible to make someone fall in love with you within hours only. The "love" test involves questions and ends with two people looking at each other's eyes for four minutes.
Penny and Sheldon begin the test. They have agreed that a dinner with Robert Downey Jr. is cool, and neither of their perfect days involve their significant others. As soon as Penny began to answer the series of questions, Sheldon expressed that she is not avoiding intimacy and that her interest is in reading people's minds.
"Well, now I wish I had the ability to make that stuff easier for you," Penny said.
As Penny and Sheldon progress, the latter admits that it is actually his birthday. He has kept his birthday a secret and Penny immediately promised to keep it like that.
Both enter the last part of their test, staring into one another's eyes for four minutes. Sheldon and Penny find it weird, though Sheldon did not have trouble maintaining an eye contact with Penny. He definitely feels comfortable with her.
When the time was up, Sheldon concludes that the experiment is a failure because they did not fall in love with each other; however, they did admit that they feel closer.
Penny walks Sheldon to his apartment where the gang prepared a surprise birthday gathering for Sheldon.