Penny asks Sheldon for advice on whether or not she should date Leonard, and during the conversation, Sheldon corrects her use of physics terms. He then explains the Schrödinger's Cat thought experiment to help her think about the situation.
Sheldon is in the car on a road trip with his dad and brother. They are talking about roadkill, and his dad asks if it's alive or dead if it's still twitching. Sheldon mentions that it sounds like Schrodinger's Cat and explains the theory.