We're still going to get "but getting an eye doesn't make a fish give birth to a cat". A thread that explains it all simply enough to a creationist is going to be about as large as one of Dawkins' books. (It actually takes a few of his books to fully explain evolution.)
One thing they should be made aware of is that "species" is an arbitrary division invented by man - it's not like the dividing line between land and ocean. Are the donkey and the horse different species? The lion and tiger? Simple question, complex answer.
And would anyone want to post a couple of examples of ring species?