How do I create a solid that is filled w mass inside?

When I create solids and delete a surface the inside is hollow. How can I create a true solid box that is filled so that it is not hollow I inside when I cut it.
Thank you.

I think reading this will help:

To answer your question, you would create a surface that matches the edges is what you sliced off, and Join it together to make a closed, solid, polysurface again.

I assigned your message to the Rhino for Windows category.

To cut a solid with a curve use WireCut.
To cut it with a surface or polysurface (including solids) use BooleanSplit or other Boolean operations.

That’s not really right. “Solid” modeling is just “surface” modeling where the system just doesn’t LET you do anything that would leave the model open.