Obvious is the opposite of Surprise. Surprise is related to Supply. 
Lean is a synonym of Tend. Tendency is derived from Tend. Tendency is a type of Direction. Direction is related to West. 
Obvious <--Antonym--> Surprise <--RelatedTo--> Supply 
Lean <--Synonym--> Tend <--DerivedFrom-- Tendency --IsA--> Direction <--RelatedTo--> West 