Round is the opposite of Square. Square is related to Bread. 
Birthday is the opposite of Happy. Happy is related to Cheer. Cheer is a synonym of Assure. Assure is a synonym of Convince. 
Round <--Antonym--> Square <--RelatedTo--> Bread 
Birthday <--Antonym--> Happy <--RelatedTo--> Cheer <--Synonym--> Assure <--Synonym--> Convince 