The American population grew from less than 4 million in the late 1700s to nearly 300 million today due to immigration and increased birthrates. There are two ways to become a US citizen - by birth or through naturalization. Citizenship by birth means being born anywhere in the US or having US citizen parents even if born abroad. Naturalization requires filing paperwork, living in the US for 5 years, being 18 years old, passing a test, and taking an oath. Legal aliens have permission like visas to be in the US but cannot vote or hold office, while illegal aliens cannot do anything legally and can be deported if found.