Here's why:
* Prepositional Phrase: It consists of a preposition ("by") and its object ("side").
* Adverb: It modifies a verb, adjective, or another adverb by describing how, when, where, or to what extent something happens.
For example:
* "The children walked side by side." (modifies the verb "walked")
* "The houses stood side by side." (modifies the verb "stood")
* "They were working side by side." (modifies the verb "working")
Let me know if you have other phrases you'd like to explore!