  A fascinating property of extremal Kerr black hole (BH) is that it could be
act as a particle accelerator with infinite high center-of-mass (CM) energy
\cite{bsw}. In this note, we would like to discuss about such fascinating
result and to point out that this infinite energy at the event horizon comes
solely due \emph{to the singular nature of the extremal limit}. We also show
that a non-extremal Kerr BH can \emph{not} transform into extremal Kerr BH by
the Ba\~{n}ados-Silk-West mechanism. Moreover, we discuss about three possible
geometries (near extremal, purely extremal and near horizon of extremal Kerr)
of this mechanism. We further prove that near extremal geometry and near
horizon geometry, precisely extremal geometry of extremal Kerr BHs are
qualitatively different. Near extremal geometry and near horizon geometry gives
the CM energy is finite, whereas precisely extremal geometry gives the
diverging energy. Thus, we can argue that extremal Kerr BH and non-extremal
Kerr BH are quite distinct objects. Finally, we show that the CM energy of
collisions of particles not only diverges at infinite red-shift surface
($r_{+}$) but it could also diverges at the ISCO ($r_{isco}$) or at the
circular photon orbit ($r_{cpo}$) or at the marginally bound circular orbit
($r_{mbco}$) or at the Cauchy horizon i.e. at $r \equiv
r_{isco}=r_{cpo}=r_{mbco}=r_{+}=r_{-}=M$.
