As I understand it, the evidence for the basic framework of evolution is quite strong. Perhaps this would be a good example for seeing what makes scientific evidence "unnatural" and hard to follow or believe. The scientific evidence is pretty overwhelming for evolution, yet many people resist it despite the evidence. A closer investigation of why this is so might reveal just the sort of difficulties people have with scientific arguments, that Eric was talking about.

Maybe we could start by collecting together the pieces of evidence people know about: the fossil record and age determinations of fossils, the mechanisms of mutation, etc.

Here's one source of resources on this:
http://www.aaas.org/spp/dser/evolution/science/default.htm

Todd