Environmental Animation - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 364

It would be cool if the petals of the flowers in the vase reacted to the same wind that affects the shears.
To add a soft body simulation, we first have to define which portions of the flower mesh will be simulated
by creating a goal vertex group. To the soft body simulator, the “goal” is the shape it tries to maintain.
Within the vertex group we are about to create, a value of 1.0 (red in the Weight Painting interface)
represents 100% of the goal, indicating that the soft body simulator should not even bother with that part
of the mesh. It needs to remain 100% in its modeled state. Weight paint that down to 0.0, though, and
the simulator takes over full responsibility for the mesh's motion.
With the flower selected, bring up the Mesh properties and go into Edit mode. Create a new vertex group
called “goal,” select all vertices in the mesh, and press the Assign button on the Vertex Groups panel.
You've just added the entire flower to the “goal” group at 100%. Adding a soft body simulation at this
point would do nothing, because the “goal” group would not let it handle any of the mesh. Switch from
Edit mode to Weight Painting mode, either with the 3D view header menu or by pressing first the Tab
key, then Ctrl-Tab. The entire flower should appear red. If it doesn't, go back to Edit mode and make
sure that the Weight control on the Vertex Groups panel is set to 1.0, instead of 0.0. Set it to 1.0, and
reassign the vertices to the group.
Using the standard Weight Painting interface you've used before, set the painting weight to 0.0 and paint
the flower petals blue. The result is shown in Figure 13.7 . Any area painted blue will be ruled by the soft
body simulator. If you want your entire object to receive the soft body treatment, you just skip the entire
vertex group creation process.
Figure 13.7 Weight painting a goal vertex group onto the lower.