At the ending, when Raymond finally figures out the punchline of "Who's on First?".
Raymond, who (like many born with autism) is phobic about being touched, as Charlie found out repeatedly through the movie. But at the end, he leans his head on Charlie's, and spells his name...the closest he can ever come to saying "I love (and trust) you." (Of course, it's also a monster of a Tear Jerker...)