I attended a birthday party in 1991. My teacher was then 59 years old. But her fellow teacher said that we were celebrating her 39thbirthday! We were all surprised and puzzled. After the party I was told by the fellow teacher that American women have a special saying about birthdays. If a woman is over 40, or even if she is 80, her birthday is always the 39th. If she is in her thirties, her birthday will be the 29th. And in her twenties, it's the 19th. They usually have their "real" birthday when they are below 20. This shows that American women, especially elderly women, are very sensitive  about their ages. But my teacher always says, "We want to be younger," so about the age of Western women, my advice is that if you don't know it, don't ask about it; if you really know it, don't mention it. The writer felt the birthday party he attended was   _  .
very interesting very strange very common very sensitive