Contributed by B@rt.
You can think of meta balls as magnetized liquid
metal. They can both repel and attract each other,
blend and dissolve each other. Meta balls are created
by pressing space and then selecting MetaBall.
Always remember that when you create a metaball it
starts in edit mode. This is so you can easily adjust
the complexity of it. If you scale a metaball inside
editmode it will grow more complex but not if you
scale it outside of editmode. Blender is capable
of creating regular and negative metaballs and
regular and negative meta tubes. Meta tubes are a
meta ball stretched at the centre to form a tube. This
tutorial will go through all forms of metaballs in Blender.
[taken from https://www.rash.f2s.com/tutorials/duck/duck.html