Sigh is related to Sad. Sad is related to Sack. Sack is related to Brown. Brown is related to Tree. 
Oil is a type of Fluid. Oil is related to Mud. 
Sigh <--RelatedTo--> Sad <--RelatedTo--> Sack <--RelatedTo--> Brown <--RelatedTo--> Tree 
Fluid <--IsA-- Oil <--RelatedTo--> Mud 