Rat is a type of Pad. Pad is a type of Leaf. Leaf is related to Tree. 
Useful is related to Note. Note is related to Bill. Bill is related to Must. 
Rat --IsA--> Pad --IsA--> Leaf <--RelatedTo--> Tree 
Useful <--RelatedTo--> Note <--RelatedTo--> Bill <--RelatedTo--> Must 