Exercise 1: Root Module vs. Child Module, and Why Every Prior Configuration Was Already a Module — Possible Solution
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The ROOT MODULE is whichever directory of `.tf` files `terraform`
commands are actually run FROM -- it's the entry point, the one
Terraform reads first when you run `init`/`plan`/`apply`. A CHILD
MODULE is any other directory of `.tf` files called via a `module`
block, either from the root module or from another child module. The
distinction isn't about what's INSIDE a module (both are just
directories of `.tf` files using the same language) -- it's purely
about WHICH ONE is the starting point Terraform was invoked against.

Per the chapter's own definition, "any directory of .tf files is a
module." Every single configuration built across Chapters 1 through 6
of this course -- the `local_file` example, the variables/locals
chapter, the state chapter's examples -- was a directory of `.tf`
files that `terraform` commands were run directly against. That
description exactly matches "root module": nothing in the definition
requires a configuration to CALL another module in order to itself
qualify as one. Every configuration in this course was, from the very
first chapter, already a module -- specifically the root module of its
own tiny, single-directory setup, simply one that never happened to
call any child modules of its own.

WHY THIS WORKS AS AN ANSWER
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This correctly identifies that the root/child distinction is about
INVOCATION POINT, not internal complexity, and then applies the
chapter's own "any directory of .tf files is a module" definition
directly to the prior chapters' examples to show they always already
satisfied it, rather than treating "root module" as a special new
category introduced only in this chapter.