If someone tells you something and you're not sure it's entirely true, it's natural to try to consider a more plausible explanation. For example, a boy doesn't present his homework in class, instead saying that it was eaten by his dog, just as he left for school. He produces a note from his Dad confirming the story. He has evidence to support his claim, but nobody really thinks the story is true. A more likely story would be that he didn't do his homework, then his Dad wrote him a note as he doesn't care about the homework and doesn't want to see his son getting into trouble.

At this point we have two versions of events - one is backed with evidence, the other is backed by common sense and probability.

If at this point you believe that he didn't do the homework, you've subscribed to a conspiracy. This means that you're paranoid, an egomaniac, you think you're cleverer than everyone else, you can't live in the real world so you have to invent stories to feel better about yourself, and so on, and so on.

This was a true story and the boy didn't even have a dog. His Dad hated school, teachers and homework and sent notes in regularly to excuse his son from anything.

When we were told that Iraq had weapons of mass destruction or "WMD!", many people felt that this was a lie and an excuse to start a war. They were the crazy conspiracy theorists. Now it's plainly obvious that this was a lie and that no such weapons existed. The crazy people were right, we ignored them and now thousands of innocent men, women and children all around the world have been maimed and killed.

Throughout history the people in control have lied to the rest of us, why would we think that it isn't happening today?