Nomes live ten times faster than humans. They're harder to see than a high- 
speed mouse. That's one reason why most humans hardly ever see them. The 
other is that humans are very good at not seeing things they know aren't 
there. And, since sensible humans know that there are no such things as 
people four inches high, a nome who doesn't want to be seen probably won't 
be seen. 
        -- (Terry Pratchett, Wings)