To ensure your feature is truly "solid" (water-tight for 3D printing or boolean operations):
: Select your object; the type should read "closed solid polysurface." Specific Version Note Rhinoceros 7.24.22297.11002 RC
: Select a group of intersecting surfaces or polysurfaces that "box in" a volume. Rhino will trim the excess and join the pieces into a single closed solid. To ensure your feature is truly "solid" (water-tight