31. The method of claim 29 wherein said set of sentences are extracted automatically from a corpus that includes web logs, newspapers, books and/or the Internet.
29. A method of identifying a source of data input to a computing system comprising: a) receiving speech utterance from an entity related to randomly selected challenge text; wherein said challenge text represents a selected set of one more contiguous words which when articulated have a measurable difference in acoustical characteristics between a reference human voice and a reference computer synthesized voice that exceeds a target threshold; b) processing said speech utterance with said computing system to compute first acoustical characteristics of said entity; c) generating a determination of whether said speech utterance originated from a machine or a human; and d) determining a set of sentences for inclusion as challenge text based on an articulation difficulty score computed for a computer synthesis engine.