To become a citizen of the United States, naturalizing immigrants must take a test. Many native-born Americans would fail this test. Indeed, most of us have never really thought about what it means to be a citizen. One radical idea from the immigration debate is the repeal of birthright citizenship—guaranteed by the Fourteenth Amendment—to prevent so-called anchor babies. Odious and constitutionally dubious as this proposal may be, it does prompt a thought experiment: What if citizenship were not, in fact, guaranteed by birth? What if everyone had to earn it upon turning 18, and renew it every 10 years, by taking an exam? What might that exam look like?