You can create soft bodies from:

• polygonal surfaces
• NURBS curves and surfaces, including curves for IK splines and wires
• lattices

You cannot make soft bodies from:

• IK skeletons
• anything in the underworld of a shape, such as a curve-on-surface
• trimmed NURBS surfaces, unless you convert them to polygons first
  TipThe more CVs or vertices the original geometry has, the more deformations that occur when you apply dynamics to the resulting soft body. For instance, applying a field to a polygonal cube with eight vertices per face causes less bulge and wiggle than one with 16 vertices per face.

To create a soft body

1. Select the object you want to make a soft body.
2. Select Soft/Rigid Bodies > Create Soft Body > to display the options window.
3. Set the options in the Soft Body Options window. See Soft/Rigid Bodies > Create Soft Body.