Plastic is the opposite of Metal. Metal is related to Smooth. Smooth is a type of Rid. 
Find is etymologically related to Path. Path is related to Street. you are likely to find Ball in/at/on Street. 
Plastic <--Antonym--> Metal <--RelatedTo--> Smooth --IsA--> Rid 
Find --EtymologicallyRelatedTo--> Path <--RelatedTo--> Street <--AtLocation-- Ball 