One day a neighbor came to ask to borrow Nasruddin's donkey, but Nasruddin didn't want to comply and so he said that unfortunately the donkey was not there. As the neighbor turned away there was a loud bray heard from the barn. The neighbor came back frowning and said: 'Didn't you tell me the donkey was not here?!' Nasruddin answered: 'Who are you going to believe, me or the donkey?'
I believe that when it comes to things like 9/11, the main obstacle to seeing that things just don't add up in the official investigation, is that already in school we learn to believe in 'authority', obey 'authority' and, to a degree, trust 'authority'. As a result we sometimes actually believe things that are contradicted by what we see with our own eyes.