. Productivity -- This means that human languages allow speakers to create novel, never-before-heard utterances that others can understand. For example, the sentence "The little lavender men who live in my socks drawer told me that Elvis will come back from Mars on the 10th to do a benefit concert for unemployed Pekingese dogs" is a novel and never-before-heard sentence (at least, I hope it is!), but any fluent speaker of English would be able to understand it (and realize that the speaker was not completely sane, in all probability).