"Sign Language!" is an episode of Oobi from the second season.
Oobi and Kako make a new friend in the park who seems different from them. Grampu explains that the girl is deaf, and helps Oobi and Kako learn some sign language so they can communicate with her. — NickJr.com
Oobi, Kako, and Grampu arrive at the park. Kako is holding a cup-ball toy. He tells the viewers about the cup-ball game, which he and Oobi decide to play by themselves. They find it difficult until a young girl appears and shows them how to play it. The girl does not speak whatsoever and leaves shortly afterwards. The boys are impressed by her skills and tell Grampu about her, referring to the girl as the "cup-ball queen." Grampu asks for her name, but the boys are unable to tell him as they never heard it.
Kako spots the girl sitting on a bench with her mother. Grampu notices that she is using sign language and explains to the boys that she is deaf. Oobi and Kako decide that they want to learn sign language so they can communicate with their new friend. Grampu, who knows several movements in sign language, teaches them how to wave "hello." The boys use this symbol to greet the girl. The girl signs the letters for "Amy," and her mother tells the boys that it is her name. Amy's mother shows Oobi and Kako how to sign their own names. They also learn the signs for "friend" and "play." Amy joins the boys in a game of tag.
The next scene is an interview segment in which Uma talks with a boy and his deaf mother. She learns the signs for "fish" and "yellow." The scene cuts to Oobi, Kako, and Amy. Grampu tells the boys that they need to go home. Amy shows them the sign for "goodbye." Oobi, Kako, and Amy sign "goodbye" as the episode ends.