Season 6: Episode 17 - The 'OCU' Or The 'PADO'

The 'OCU' Or The 'PADO': When Charlie tells Chelsea that he loves her, she replies with a simple "thank you." Not used to putting his feelings out there and not having them be reciprocated, Charlie goes to Dr. Freeman to talk the issue through. He finds that he needs the relationship to be on equal terms, and in order to get Chelsea to say "I love you," he decides to ask her to marry him. However, when he tries the old "engagement ring in a glass of Champagne" routine, Chelsea accidentally swallows the ring.