Example: you were talking about rocks and petrified trees, rocks are made out of minerals, but petrified trees are taken over by minerals.
An example of a compound sentence is: I wanted to scare everyone during Halloween, but my mom said i would get in trouble if i tried to. A compound sentence basicly contains two simple sentences, connected by a conjunction. The conjunctions are for,and,nor,but,or,yet. Or, FANBOY. And for something to be a conjuction, it has to have a complete sentence and a comma before it.