Make a metaball in the normal fashion. Make a second metaball but hold down the shift key while clicking on the metaball icon in the create menu; this will be your negative metaball. Move the negative metaball so that it is close proximity to the normal one, but is not inside it. (use the top, side views). You should see something like this when you render (it doesn't show up in wireframe, only when rendering).

This is not my "second childhood". I'm not finished with the first one yet.

Time flies like an arrow; fruit flies like a banana.

"I'd like to die peacefully in my sleep like my grandfather....not screaming in terror like the passengers on his bus." - Jack Handy