ad of 
&gt; sweat on my brow an OPO? It is virtually impossible to walk through 
&gt; the world during daylight without seeing several hundred things every 
&gt; hour which would be problematic to classify firmly as either OPOs or 
&gt; non-OPOs.

Actually, to my mind, a piece of thread seems to be a pretty clear case of
an ordinary physical object (albeit a rather small one).  As for paint and
sweat, I think they point out a need for subclassifying ordinary physical
objects into something like the stuff/ob