
The mouth is a negative plane
Negative plane?
meta-ball-plane-thing … or did i squash down an ellipsoid? sigh… now that you ask… 
But negative? I tried and did not see a way? Probably never touched metaballs in years though!
After adding a Metaball object we can turn it into a negative Metaball object in edit mode. It will eat into other metaballs of the same family.
exactly
Ah yes so it does.

I will find a use for them one day!
I like it very much. Since I only started sculpting, It is an excellent tool for me to make holes instead of reversing the draw tool with control or grabbing inwards.