Linguistics; ET languages...

SF writers trying to describe an ET language in their fiction tend to lean toward languages made up of colors or smells or textures or musical notes or some such thing. But a Terran language could easily work that way.

What you need to identify a language as extraterrestrial is some feature never before observed in any Terran language. For example, suppose you encounter a language that has three basic classes of meaningful sounds: vowels, consonants -- and something else. The question then is...