Obvious is the opposite of Surprise. Surprise is related to Kind. Kind is a synonym of Form. 
you are likely to find Pain in/at/on Ass. Pain is related to Cause. Cause is related to Disease. 
Obvious <--Antonym--> Surprise <--RelatedTo--> Kind <--Synonym--> Form 
Ass <--AtLocation-- Pain <--RelatedTo--> Cause <--RelatedTo--> Disease 