Collision Shapes - Learning Game Physics with Bullet Physics and OpenGL - page 116

Our new dumbbell object
It's worth mentioning that we only created two unique shapes in memory for our
dumbbell: one for the rod and one for the load. Yet, our dumbbell is built from three
unique shapes. This is an essential memory saving feature of collision shapes. Their
data can be shared by more than one collision object, and still be treated as two
unique instances.