40. The system of claim 24 where said first set of words and phrases are automatically extracted from an electronic corpus.
24. A system for identifying challenge data to be used for distinguishing between humans and machines in allowing access to computing resources, comprising: a computing system; and one or more software routines embodied in a computer readable medium adapted to cause the computing system to: (a) automatically generate a candidate challenge sentence from a first set of words and phrases; (b) automatically generate at least one first utterance from a first machine text to speech system of said candidate challenge sentence, said at least one first utterance including first acoustical characteristics; (c) acquire at least one second utterance of said candidate challenge sentence known to be from a human speaker, said at least one second utterance including second acoustical characteristics; (d) automatically determine a difference in said first and second acoustical characteristics to determine a challenge sentence acoustic score for said candidate challenge sentence; (e) automatically select said candidate challenge sentence as a suitable challenge item in an utterance-based challenge system and store said at least one second utterance and said candidate challenge sentence in a challenge item database using the computer system when said candidate challenge sentence acoustic score exceeds a target threshold, indicating that said at least one first utterance from the first machine text to speech system is sufficiently different from said at least one second utterance from the human speaker for said candidate challenge sentence to be used to automatically distinguish between humans and machines.