PZ Myers discusses some common Creationist misunderstandings about evolution, and points out that while they are often based on deep, fundamental errors of how the world works (dare I say it, stupid questions) others are not stupid at all, but reveal insight and good reasoning ability.
PZ contrasts the typical stupid Creationist question "If we evolved from monkeys, why are there still monkeys around?" with an insightful question about why animals evolved with two eyes. How did they know that one eye is not enough for depth perception and therefore they'd need two eyes?
The answer, and it's not an obvious one, is that two eyes is an accident of developmental biology. As PZ explains, paired structures (two lungs, two arms, two legs...) are a natural consequence of the way cells grown under bilateral symmetry:
It's harder to generate single structures than paired structures with that kind of symmetry, and the real question is how the anterior-posterior and dorsal-ventral axis is generated…and it's not an easy one to answer!
The real problem isn't that Creationists are stupid, because they generally aren't. Ignorant, maybe, but that's not the problem either. Millions of people have no idea of how to raise cattle for beef, but you don't see them protesting that hamburger and steaks are impossible.
PZ puts the blame firmly on arrogance:
The common theme in creationist objections, in the letters I get, in the whole damn culture war, is that creationists arrogantly assume that their ignorance is shared and that it is a valid data point in our explanations of the world. It isn't the scientists who are the arrogant ones in this debate, it's people who come out of 6th grade sunday school utterly convinced that they have all the answers.
PZ is right as far as it goes, but where does that arrogance come from? These people, as a rule, would never dream that they understand how televisions or DVD players work, or imagine that know how to repair bagpipes. Why are they so arrogant about biology?
Two factors: firstly, people have deep intuitions about other living creatures. That folk biology is a set of heuristics that enabled our ancestors to make a good living predicting the behaviour of the natural world over the short-term period of a generation or two. Consequently, we have a deep inherent prejudice for trusting that dogs will beget more dogs. This is so strong, that it took mankind thousands of years to come up with the idea of evolution, even while they were busily using selective breeding to evolve domestic plants and animals from their wild ancestors.
While we might have an inherent prejudice for folk biology, our intellects can over-ride that. And that's were the second factor comes in. Religion. Some religious memes give people deeply felt emotional needs that conflict with facts (be they facts of biology or anything else). Other religious memes give people the totally undeserved certainty that they know the Truth, the Whole Truth and Nothing But The Truth.
Combine the heady mix of false certainty and emotional commitment given by religion with ordinary garden-variety ignorance, plus the human prejudice for folk biology, and you get the Creationist arrogance about science.