Purchase is related to Afford. Afford is related to Buy. Buy is related to Trading. 
Car is related to Oil. Oil is related to Mud. Mud is a type of River. River is related to Clear. 
Purchase <--RelatedTo--> Afford <--RelatedTo--> Buy <--RelatedTo--> Trading 
Car <--RelatedTo--> Oil <--RelatedTo--> Mud --IsA--> River <--RelatedTo--> Clear 