Considering how complex the human brain is, it's pretty easy to fool.  Check out this video and watch this dude's head disappear. 

 

I guess how this all works is due to a blind spot in your eyes.  It has something to do with like corneas or some other doctor thing but it's still pretty sweet to see a dude without a head.  You experience these blind spots all throughout your day, but your brain fills in the blanks for what your eyes miss.