Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. They usually indicate location, time, or direction.
* Since is a conjunction (specifically a subordinating conjunction). It connects a dependent clause to an independent clause, showing a cause and effect relationship or a time relationship.
Here are examples of how the other words function as prepositions:
* Or: I will go to the park *or* the beach. (This is a coordinating conjunction, but it can also function as a preposition in some cases.)
* Above: The bird flew *above* the trees.
* Around: We walked *around* the lake.
Let me know if you'd like more examples of prepositions!