Why do people see UFOs in so many different shapes? Is it simply human psychology interpreting strange things in ways which match familiar things? Or is it perhaps because what we see in the sky may arise from a variety of different causes?

Skeptics laugh at the idea of credibility. Everyone can make mistakes, they argue, and UFOs tend to be the biggest mistakes of all. But are there groups of people who might be more credible as witnesses than others? How should we measure credibility?