Airplane can be used for Travel. Airplane is related to Case. Case is related to Half. Half is related to Ocean. 
Density is related to Oil. Oil is related to Used. Used is a different concept from Fresh. Fresh is related to Quality. 
Travel <--UsedFor-- Airplane <--RelatedTo--> Case <--RelatedTo--> Half <--RelatedTo--> Ocean 
Density <--RelatedTo--> Oil <--RelatedTo--> Used <--DistinctFrom--> Fresh <--RelatedTo--> Quality 