As the elements interact with each other, four different kinds of relationship between them are possible. Metal and Metal controls Wood. Earth and Earth insults Wood. The best way to visualise these first two cycles is as a mother and child. So, Wood is the child of Water and the mother of Fire.
Every element controls another and is controlled by one itself.
This maintains the natural balance in the system. When the harmony is lost and the controlling element becomes too strong, the controlled element can be destroyed. The insulting sequence is the reverse of the controlling sequence. The insult can only happen when the system is out of balance. If an element becomes too weak it can be insulted by the element it normally controls.