Of the roughly seven thousand languages spoken today, linguists estimate that about half will have no native speakers left by the end of the century. The immediate cause is rarely coercion. More often, parents in small communities make a reasonable calculation: schooling, employment and official business are all conducted in a dominant language, so they raise their children in it instead. What vanishes with a language is not merely a set of words. Systems for classifying plants, for navigating by the stars or for tracking seasonal change are encoded in vocabulary with no equivalent elsewhere, and translation preserves only part of what they hold.