Why do cold engines idle slowly?

[jhoward@argus.lowell.edu (James D. Howard II)]

A coworker and I were wondering why an engine idles more slowly when
it is cold than when it warms up.  I thought it was because there is
more internal friction when it is cold, due to thicker oil, and the
pieces not having expanded to their full size.  He thought it was
because the air fuel mixture does not burn as completely inside a cold
cylinder as in a hot one.  So which is it?  Or is it a little of both?

James