filling a volume

If I want to create a mesh like a cup and then fill the volume with another mesh what is the easiest way? Could I select the inner vertices and then do some kind of invert command?
thanks in advance,

You could select the faces on the inside of the cup from the bottom to as high as the liquid goes, duplicate them (Shift D), then scale them slightly smaller. From there you could close off the top with the fill command, separate it into a separate mesh, or just texture it separately.

Or if you wanted liquid and accuracy then try the fluid simulator