For example, the popular metaphor in which the propagation of electricity through wire is conceived as the flow of water through pipes is certainly useful when people first learn 
(the propagation of electricity through wire; is conceived; as the flow of water through pipes; L:the popular metaphor) (the popular metaphor in which the propagation of electricity through wire is conceived as the flow of water through pipes; is certainly; useful; T:when people first learn) (people; learn; T:first)
the propagation of electricity through wire the popular metaphor in which the propagation of electricity through wire is conceived as the flow of water through pipes people
is conceived is certainly learn
as the flow of water through pipes L:the popular metaphor
useful T:when people first learn
T:first