Its the best response to the typical "if evolution is true where are all the transitionals?" question.
Just ask "well first what do you think a transitional would look like?"
And immediately they try and escape the topic for they know if they actually make their claim testable, ie put their ss on the line, it could very well be refuted.
A transitional fossil would probably look like a member of a ring species, but that wouldn't do either, because ring species are in transition right now, and aren't fossils.