Here's why:
* Subject: The subject of a sentence is the person, place, thing, or idea that the sentence is about.
* Simple Subject: The simple subject is the main noun or pronoun that performs the action of the verb.
In this case, the sentence is about a bolt of lightning and what it can do. The word "bolt" is the main noun performing the action (melting). "Lightning" is part of a noun phrase describing the bolt.