Is there really any need to explain this?
Charlie Chaplin historian JeTamme Derouet provides some interesting background info for this photo and describes the pair meeting in Hollywood during one of Miss Keller's speaking tours:
"They spent most of the day off alone together and they smiled and laughed a lot like they had their own private world, just walking the studio lot and talking privately together. (The picture seen here) is of her holding her hand to his mouth to feel what he is saying as he speaks to her."
To paraphrase Derouet on Chaplin's interest in sign language:
"Chaplin had worked with several deaf people throughout his career in an effort to improve his own pantomime communication skills and traveled with deaf artist Granville Redmond, who taught Charlie sign language and fingerspelling. He was also a great supporter of the deaf during a time when most people in America were trying to abolish sign language all together."
Ridiculously good guy Chaplin was a ridiculously good guy.