CBS

Season 3: Episode 15 - The Large Hadron Collision

The Large Hadron Collision: Leonard is invited to Switzerland on Valentine's Day to see the CERN Supercollider, the world's largest and highest-energy particle accelerator, and Sheldon is shocked when Leonard decides to take Penny, making it a romantic Valentine's Day present, instead of him. Sheldon tries several angles in an effort to change Leonard’s mind including guilting Penny into not going, but Leonard isn’t budging and tells Sheldon that he would rather go alone than take him. Unfortunately for Leonard, Penny gets sick the night before they have to leave so Leonard decides to make peace and invite Sheldon to go. When Leonard gets to the apartment, he finds Sheldon is horribly sick too. Penny and Sheldon end up staying home sick together and Leonard is forced to take Koothrappali to Switzerland for what was supposed to be a romantic getaway.