  The ubiquity of very thin and lengthy cold neutral media (CNM) has been
reported by multiple authors in the HI community. Yet, the reason of how the
CNM can be so long and lengthy is still in debate. In this paper, we recognize
a new type of instability due to the attractive nature of the pressure force in
the unstable phase. We provide a new estimation of the average CNM filament
aspect ratio with the consideration of force balances at the phase boundary,
which is roughly 5-20 in common CNM environment. We show that most of the cold
filaments are less filamentary than what usually predicted via MHD turbulence
theory or inferred from observations: The average length of CNM filament is
roughly 1/2 of that in isothermal MHD turbulence with similar turbulence
conditions. This suggests that the "cold filaments" that is identified in
observations might not be in pressure equilibrium or generated via other
mechanisms.
