  The vorticity of world lines of observers associated to the rotation of a
massive body was reported by Lense and Thirring more than a century ago. In
their example the frame dragging effect induced by the vorticity, is directly
(explicitly) related to the rotation of the source. However in many other cases
it is not so, and the origin of vorticity remains obscure and difficult to
identify. Accordingly, in order to unravel this issue, and looking for the
ultimate origin of vorticity associated to frame dragging, we analyze in this
manuscript very different scenarios where frame dragging effect is present.
Specifically we consider general vacuum stationary spacetimes, general
electro-vacuum spacetimes, radiating electro-vacuum spacetimes and Bondi--Sachs
radiating spacetimes. We identify the physical quantities present in all these
cases, which determine the vorticity and may legitimately be considered as
responsible for the frame dragging. Doing so we provide a comprehensive
physical picture of frame dragging. Some observational consequences of our
results are discussed.
