Station is a synonym of Stop. Challenge entails/necessarily involves Stop. Challenge is related to Difficult. 
Flip is a synonym of Pitch. Key is a type of Pitch. Key is related to Note. Note is related to Book. 
Station <--Synonym--> Stop <--Entails-- Challenge <--RelatedTo--> Difficult 
Flip <--Synonym--> Pitch <--IsA-- Key <--RelatedTo--> Note <--RelatedTo--> Book 