System can be used for Efficiency. System is a type of Method. 
Leg is related to Furniture. Furniture is related to Table. Table is related to Related. Related is related to Temperature. 
Efficiency <--UsedFor-- System --IsA--> Method 
Leg <--RelatedTo--> Furniture <--RelatedTo--> Table <--RelatedTo--> Related <--RelatedTo--> Temperature 