- Languages change over time through natural processes like sound changes in casual speech that become conventionalized, as well as language acquisition by children and language contact between groups. - The rate of change varies, but after 1,000 years languages become mutually unintelligible, and after 10,000 years the relationship is indistinguishable from unrelated languages. - Comparative methods are used to reconstruct proto-languages and classify language families by examining regular sound correspondences between related languages. For example, Grimm's Law describes consonant changes between Proto-Indo-European and Proto-Germanic languages.