Where does one race end and another begin?

http://racerelations.about.com/od/understandingrac...

"Thanks to scientific advances, we can now say definitively that individuals such as Morton and Aggasiz are wrong" (which basically they said that race is described by brain size, color, facial/hair characteristics). "Race is fluid and thus difficult to pinpoint scientifically." “Race is a concept of human minds, not of nature,” Relethford writes.

Briefly, an AA or Black American can look Chinese and some chinese can have European facial features, so if race is "brain size, color, facial/hair characteristics" where does the little boy who looks black with european facial features be categorized? Or the little girl that is Asian but looks european be categorized?

"there’s a high degree of gene flow among people groups that makes it hard to organize them into discrete groups." Says the article I linked above.

I have full lips, small nose, curly hair, and make good grades excluding my ditsy ways but so far I'm *******, Mongoloid and Caucasoid, where does the line of race end and the other begin?