Air is a type of Gas. Gas is a synonym of Fuel. Candle is a type of Fuel. Candle is related to Old. 
Barrier is a synonym of Obstacle. Obstacle is a synonym of Difficulty. Difficult is etymologically related to Difficulty. 
Air --IsA--> Gas <--Synonym--> Fuel <--IsA-- Candle <--RelatedTo--> Old 
Barrier <--Synonym--> Obstacle <--Synonym--> Difficulty <--EtymologicallyRelatedTo-- Difficult 