Sky is the opposite of Water. Water is related to Fog. Fog is related to Head. Head is a synonym of Topic. 
Sink is a synonym of Bury. Bury is related to Ground. Ground is related to Railroad. 
Sky <--Antonym--> Water <--RelatedTo--> Fog <--RelatedTo--> Head <--Synonym--> Topic 
Sink <--Synonym--> Bury <--RelatedTo--> Ground <--RelatedTo--> Railroad 