  {Filaments are ubiquitous in the interstellar medium as recently emphasized
by Herschel, yet their physical origin remains elusive} {It is therefore
important to understand the physics of molecular clouds to investigate how
filaments form and what is the role played by various processes such as
turbulence and magnetic field.} {We use ideal MHD simulations to study the
formation of clumps in various conditions including different magnetization and
Mach numbers as well as two completely different setup. We then perform several
analysis to compute the shape of the clumps and their link to velocities and
forces using various approaches.} {We find that on average, clumps in MHD
simulations are more filamentary that clumps in hydrodynamical simulations.
Detailed analyses reveal that the filaments are in general preferentially
aligned with the strain which means that these structures simply result from
the strech induced by turbulence. Moreover filaments tend to be confined by the
Lorentz force which therefore lead them to survive longer in magnetized flows.
We show that they have in all simulations a typical thickness equal to a few
grid cells suggesting that they are primarily associated to the energy
dissipation within the flow. We estimate the order of magnitude of the
dissipation length associated to the ion-neutral friction and conclude that in
well UV shielded regions it is of the order of 0.1 pc and therefore could
possibly set the typical size of non self-gravitating filaments.} {Filaments
are ubiquitous because they are the results of the very generic turbulent
strain and because magnetic field help to keep them coherent. We suggest that
energy dissipation is playing a determinant role in their formation.}
