A child can be a U.S. citizen without a form, an oath, or any awareness of the fact, because citizenship passes from parent to child by operation of law along two very different tracks. This article maps both: acquisition at birth abroad under INA sections 301 and 309 (the jus sanguinis rules keyed to birth date, marital status, and a citizen parent's physical presence, codified at 8 U.S.C. sections 1401 and 1409), and derivation after a parent naturalizes under INA section 320 and the Child Citizenship Act of 2000. It works through the date-specific physical-presence formulas, the special and formerly gendered rules for children born out of wedlock, the Supreme Court's equal-protection line from Miller v. Albright through Nguyen v. INS to Sessions v. Morales-Santana, adopted children, retention traps, and the effective-date cliffs that make the calendar dispositive. It then explains why status and proof are different problems and how a Consular Report of Birth Abroad, a U.S. passport, and a Certificate of Citizenship on Form N-600 each function. Invented hypotheticals illustrate how the day-counts play out, and the piece closes with a decision framework, common errors, and the high stakes of an undocumented derivative citizen facing removal.