  Molecular hydrogen clouds are a key component of the interstellar medium
because they are the birthplaces for stars. They are embedded in atomic gas
that pervades the interstellar space. However, the details of how molecular
clouds assemble from and interact with the atomic gas are still largely
unknown. As a result of new observations of the 158~$\mu$m line of ionized
carbon CII in the Cygnus region within the FEEDBACK program on SOFIA
(Stratospheric Observatory for Infrared Astronomy), we present compelling
evidence that CII unveils dynamic interactions between cloud ensembles. This
process is neither a head-on collision of fully molecular clouds nor a gentle
merging ofonly atomic clouds. Moreover, we demonstrate that the dense molecular
clouds associated with the DR21 and W75N star-forming regions and a cloud at
higher velocity are embedded in atomic gas and all components interact over a
large range of velocities (20 km/s). The atomic gas has a density of 100
cm$^{-3}$ and a temperature of 100 K. We conclude that the CII 158 $\mu$m line
is an excellent tracer to witness the processes involved in cloud interactions
and anticipate further detections of this phenomenon in other regions
