COLREGs compliance and collision avoidance with a single vessel on our starboard side seems to be a very simple task. But the situation changes dramatically when the number of the scene participants increase. Bridge watchkeeping is a very situational task: the same places may be nice and quiet one day and absolutely disastrous another day. When it comes to the dense traffic and decision making, do we really understand what are we doing on the bridge and use our RADAR/ARPA equipment or we just follow some sort of our inner voice and try to match the desired CPA? And if we do, we might be really close to a trouble we unaware of. Because to know what we are doing we have to understand the principles of relative motion and radar plotting.