Here's why:
* Prepositions: Words like "for," "to," "of," "with," etc., that introduce a prepositional phrase.
* Prepositional phrase: A group of words that includes a preposition and its object (the word or phrase that the preposition refers to). In this case, "aid" is the object of the preposition "for."
Let's see an example:
* "He went to the store for aid."
Here, "for aid" tells us the purpose of going to the store.