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 couldn't
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 didn't
understand what was going on, and really being genuinely stupid. He was
renowned 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 Hitchhiker's Guide to the Galaxy_