Pillow is related to Cover. Cover is the opposite of Naked. Naked is the opposite of Wear. 
Smooth is a synonym of Flat. Flat is the opposite of Mountain. Mountain is related to Cliff. 
Pillow <--RelatedTo--> Cover <--Antonym--> Naked <--Antonym--> Wear 
Smooth <--Synonym--> Flat <--Antonym--> Mountain <--RelatedTo--> Cliff 