Things usually are exactly what they appear to be - "if it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck" - but there are indeed several categories in which we can expect this not to be true. For example, if something seems to good to be true, it probably is.
(For bonus points, try convincing someone to apply that last maxim to mainstream religion or, for something that would be more controversial here, anyone reading this who expects a "Rapture of the Nerds" to occur in their lifetime.)
I was just wondering. Human minds are messed up in 1001 ways, but are there a few rational principles that most people already have down? Of course, the answers to this question are probably so extremely obvious that I haven't even considered them. But I ask all the same.