https://en.wikipedia.org/wiki/Vietnamese_name

Father: Once upon a time, there was an emperor whose family name was Nguyễn. The people whom he ruled over wanted to show their loyalty to him, so they all changed their family names' to "Nguyễn."  Now, those people have had kids and their kids have had kids. However, you can still find the place where 40% of the citizens have the same name.

Son: Really?!  Where is it?  Can we go there?!  

Father: Yes, we can go to Vietnam, son.