One of the major difficulties Trillian experienced in her relationship with
Zaphod was learning to distinguish between him pretending to be stupid just
to get people off their guard, pretending to be stupid because he couldnt
be bothered to think and wanted someone else to do it for him, pretending
to be so outrageously stupid to hide the fact that he actually didnt understand
hat was going on, and really being genuinely stupid.  He was reknowned for 
being quite clever and quite clearly was so -- but not all the time, which
obviously worried him, hence the act.  He preferred people to be puzzled
rather than contemptuous.  This above all appeared to Trillian to be
genuinely stupid, but she could no longer be bothered to argue about.
-- Douglas Adams, _The Hitchhikers Guide to the Galaxy_