I realize alot of science is now being brought in. Fire, metal conductor, air expanding with heat etc. And Ferrum is much smarter than he seems :p Also the saying is from the chinese version of elements. 
 
Constructive:Metal generates water; water nourishes wood; wood feeds fire; fire creates earth/ash, earth bears metal 
Destructive: Fire melts metal; metal chops wood; wood break up earth; earth absorbs water; water quenches fire.