Curtain is a type of Book. Book can be used for Income. Income is related to Tax. Tax is related to Obligation. 
Steep is a type of Soak. Steep is a synonym of Plunge. Plunge is a type of Drop. Drop is related to Measurement. 
Curtain --IsA--> Book --UsedFor--> Income <--RelatedTo--> Tax <--RelatedTo--> Obligation 
Soak <--IsA-- Steep <--Synonym--> Plunge --IsA--> Drop <--RelatedTo--> Measurement 