Please HELP: need routine to select the face of a solid

Hi everibody, I'm a novice user of AUTOCAD and I'm starting learning about
AUTOLISP and Autocad customization...I need a

routine to select the face of a solid and make it become an entity (to find
the centroid of that surface then, that's what I

really need)...
I thought the way to unambiguously identify one of the solid's faces could
be by selecting 2 of their sides, but I've no idea

of how to translate it in AutoLISP.

Any help will be very appreciated

Thanks a lot

Alberto (preparing his mechanical engineering final thesis)