Matt Yglesias' expresses bafflement on how "that depends what the definition of is is" came to be a definitive moment of Clinton slipperiness. Although he makes a very good point, if my memory serves me right there's another point he missed. The statement "there is no sexual relation" wasn't even said by Clinton, it was said by one of his lawyers. The reason i remember this is that Clinton's next sentence after "that depends what the definition of is is" was the much more brilliant "usually lawyers are asked to explain their clients statements and not vice-versa" (roughly, i haven't tracked down the original quotes). So the slipperiness of the original claim wasn't directly Clinton's fault.