Gap is a synonym of Crack. Crack is a type of Opening. Neck is a type of Opening. Neck is related to Rub. 
Challenge is related to Difficult. Difficult is related to Test. Test is related to Experiment. 
Gap <--Synonym--> Crack --IsA--> Opening <--IsA-- Neck <--RelatedTo--> Rub 
Challenge <--RelatedTo--> Difficult <--RelatedTo--> Test <--RelatedTo--> Experiment 