I am absolutely no expert on Thai culture, but I do wai first on lots of occasions. maybe I am wrong but this is how I feel good about it. - the obvious case is when I am on gate duty and I wai all the 200 or so people who pass me by (such a waste of time) - if I meet someone higher up the social ladder - greeting a doctor, immigration official, policeman in the traffic fines department - if I enter someone's house as a guest, no matter what their age, or get introduced to people who are the same age as me or older (e.g. colleague's parents come to school for some reason or we need to attend a wedding / funeral in a colleague's family) - if someone does a favour for me or goes out of their way to help - for example, run around to try to change a 500 baht note for me, agree to cook something that is not on the menu, pick up stuff I dropped, lets me leave luggage in the guesthouse at no charge until departure, answers a million stupid questions and arranges a flight / visa / whatever against the odds, allows me to borrow something expensive or important such as a camera, etc. situations when in English you genuinely say "oh thank you very much", not just an automatic "thank you" with no real meaning. - if someone hands over money - not change in the supermarket, but salary in the school office, parent paying for a book, borrowing money from a colleague (even if 20 baht) or getting such loan back from them.