Poem can be used for Reading. Poem is a type of Art. Art can be used for Education. Homework is part of Education. 
Plot is related to Device. Guard is a type of Device. Guard is related to Defensive. 
Reading <--UsedFor-- Poem --IsA--> Art --UsedFor--> Education <--PartOf-- Homework 
Plot <--RelatedTo--> Device <--IsA-- Guard <--RelatedTo--> Defensive 