The Bath Item Gift Hypothesis

"The Bath Item Gift Hypothesis" is the eleventh episode of the second season of The Big Bang Theory, and the first Christmas-themed episode, originally aired on December 15, 2008. It's considered the most popular and precious episode in The Big Bang Theory's history, 'cause this was first time Sheldon had physical contact with Penny.

Synopsis
Penny informs Sheldon that she intends to give gifts to him and Leonard on Christmas which happens to be around the corner. After considerable debate with Penny, Sheldon agrees to also give a gift to Penny on Christmas. However, in the dilemma of presenting Penny with a correctly valued gift, Sheldon buys multiple basket sizes of the same bath item gifts, intending to gift the correct size according to Penny's gift and return the rest of the baskets for a full refund. At the same time, Leonard starts working with David Underhill (Michael Trucco) — the winner of the MacArthur Genius Grant — who starts dating Penny. Seeing Penny get interested in Science because of David, Leonard starts to get jealous and ends up confronting Penny about his feelings. Penny then informs Leonard of her breakup with David after she found naked pictures of his wife on his cellphone when he tried to snap naked pictures of Penny.

After Penny gets a little angry with Leonard for being so jealous about David Underhill, she eventually gives Leonard a kiss on the cheek and lays her head on his shoulder in a tender Christmas moment between the two of them. On Christmas Eve, Penny gives Sheldon a personally autographed napkin from Leonard Nimoy that he used to wipe his mouth, addressing Sheldon directly by his first name as a gift. After realizing the value of the gift and the DNA ("All I need is a healthy ovum and I can grow my own Leonard Nimoy!"), Sheldon excuses himself to get his gift for Penny. Penny then gives Leonard a gift certificate to motorcycle lessons — since it was the reason why Leonard hurt his leg — and Leonard gives Penny 101 Scientific Experiments for Kids — for her fake interest in Science while dating David. Sheldon returns to give Penny all the bath item baskets he bought for her. Since these were still not enough by Sheldon's standard, he proceeds to give Penny an awkward hug.