What secret activity does Cassia ask Ky to teach her?

When Cassia looks at him, she notices he is doing something strange. When she asks him what he is doing, he says that he is writing. It's an old-fashioned type of script. She asked how he learned to do it since they don't teach it in the schools. He says that it is his secret. He knows her secret, and now she knows his. Cassia asks if he will teach her how to write, and he reminds her that they are not supposed to do it.