In a book I am reading, the protagonist says, "Sometimes when you open the box, the cat is alive. Sometimes it's dead."
Leonard explained Schrodinger's Cat theory to Penny in an episode; otherwise when reading the main character's remark, I would have been like: "Whaaat? Where did a cat in a box come from? Did I miss something?" And then I would have been looking for previous mention of a cat in a box.