The Large Hadron Collision
The Big Bang Theory Season 3 Episode 15 (02/08/10)
Valentine's Day is coming up and when a professor pulls out his back, Leonard is given a free trip to Switzerland to see the Large Hadron Collider on February 14th. Naturally, Leonard plans on taking Penny and delivers the exciting news, while an angry Sheldon insists that based on the roommate agreement, Leonard take him. When being a jerk doesn't work, Sheldon attempts to make Leonard breakfast in bed, but still doesn't get a yes. That's when Sheldon attempts to be nice to Penny and guilt her. It seems to have worked with Penny and he even gives her a hug!
However, back at the apartment, Leonard still refuses to take Sheldon, despite his attempts to manipulate Penny. That night, Penny comes down with a flu, so Leonard goes to ask Sheldon, who caught the flu from when he hugged Penny. Leonard ends up taking Raj on his romantic Valentine's getaway.