Here's why:
* Predicate: The predicate is the part of the sentence that tells us what the subject does or is.
* Simple Predicate: The simple predicate is the verb (or verb phrase) that forms the core of the predicate.
In this case, the subject is "fire" and the predicate is "will keep them warm". The verb phrase "will keep" is the simple predicate, as it directly describes the action of the fire.