Ever since Darwin, actually even before Darwin, creationists' pet organ has been the (human) eye. They think that it is a pinnacle of complexity and impossible to explain by evolution. The fact that creationists even exist in the 21st century is a reason for communal shame, but the fact that today's creationists still invoke the eye just shows how ignorant, unimaginative and disingenuous they are. In The Origin, Darwin proposed a scenario for the evolution of the eye, and subsequent 150+ years of research have shown that he was pretty much right on the money.
If you want to catch up with the current understanding of the evolution of the eye, head over to The Loom where Carl Zimmer wrote a two-piece summary: Part I and Part II. Read it, enjoy the wonderful expository prose, bookmark it, link to it, save it, print it out, and have it ready for the next time some creationist loony imagines he has asked a Gotcha! question when he asks you about the wonderful complexity of the eye.