In short, "Best" is to interchangeable cores as "Kleenex" is to facial tissues. Although there are many other brands available, these are the two most recognizable names associated with their...
outer core is liquid where as the inner core is solid :)
inner core is solid outer core is liquid
JROTC teaches class on rank structor, also physical training so it could count as a gym class.
If the edge and corners seems to fit for example thr rubiks core works on a dayan ghong cube