In Hong Kong, Cantonese is the default language for most of the population. But there's been an increasing number of people who speak Putonghua in the country. For many Hong Kongers, this signals the increasing "mainlandisation" of the country, and a threat to a central part of their identity. Caroline Oh speaks with Lau Chaak-Ming, a linguistics expert in Hong Kong who is passionate about preserving Cantonese. Chaak-Ming is in the midst of compiling an online Cantonese-to-Cantonese dictionary at words.hk.