In a sentence or two, what makes someone Native American — and why is there no single answer?

In a sentence or two, what makes someone Native American — and why is there no single answer? (Look back at your notes.)
Blood quantum let the U.S. decide who counts as Native. Where else have you seen who 'belongs' to a group decided from outside, not by the group itself?
Still wondering one thing about Native American identity you want to learn more about.