Here's why:
* Compound Sentence: A compound sentence contains two or more independent clauses (complete sentences that can stand alone).
* Conjunction: A conjunction connects these independent clauses. Common conjunctions include "and," "but," "or," "nor," "for," "yet," and "so."
Example:
* The cat sat on the mat, and the dog chased its tail.
Without the comma:
* The cat sat on the mat and the dog chased its tail. This is grammatically incorrect and can be confusing to read.
Remember: Always use a comma before a conjunction that joins two independent clauses in a compound sentence.