Prepositions

In English today I was teaching my students about linking verbs. I explained to them that they need to put all the prepositional phrases in parentheses so they can find the subject, verb, and predicate noun or predicate adjective easier. We had just completed a sentence and I asked a student what 'of football' was? The correct answer would be 'a preposition'. Instead he begins to shape an American football with his hands and says 'it's kinda shaped like this'.