# Git Mental Model

## Three places your files can be

Git tracks your project through three distinct areas, and understanding this model is what
makes every Git command make sense — memorizing commands without it just produces confusion the
first time something doesn't behave as expected.

```
Working Tree  →  Staging Area  →  Repository (commit history)
(your actual        (what you've         (permanent,
 files, as you       marked ready          recorded
 edit them)          to commit)            snapshots)
```

- **Working tree** — the actual files on disk, exactly as you're editing them right now.
- **Staging area** (also called "the index") — a holding area for changes you've deliberately
  marked as ready to be committed. This is Git's most distinctive design choice: you don't
  commit "everything that changed," you commit "exactly what you staged."
- **Repository** — the permanent, recorded history of every commit ever made — this is what
  `git log` shows you.

## Why `git add` exists at all

A change starts in your working tree. `git add <file>` moves it (conceptually — the mechanics
are more subtle, but this framing is enough for now) into the staging area. `git commit` takes
EVERYTHING currently staged and permanently records it as a new snapshot in the repository's
history. This two-step process (stage, then commit) exists so you can build up a commit
DELIBERATELY — staging exactly the related changes that belong together, even if your working
tree currently has other, unrelated changes you're not ready to commit yet.

## Core vocabulary

- **Commit** — one permanently recorded snapshot, with a message describing what changed and
  why, and a unique identifier (a hash).
- **Branch** — a movable pointer to a specific commit, letting you develop something (a
  "feature branch") without affecting the main line of history until you're ready to combine it
  back in (Module 4).
- **HEAD** — Git's pointer to "which commit you're currently looking at" — almost always the
  tip of whichever branch you have checked out.
- **Remote** — a reference to another copy of the same repository, usually hosted somewhere else
  (Module 5).

## Instructor Notes

**Lesson goal:** Establish the three-area model BEFORE any commands — same "model before syntax"
pedagogy as ROOT 01's process model and ROOT 05's Kubernetes mental model. Every subsequent Git
command should be explainable in terms of moving something between these three areas.

**Approximate timing:** 15 minutes read; 10 minutes live, ideally with a literal diagram drawn
live (whiteboard or shared screen) showing files moving between the three areas.

**Common misconception to watch for:** assuming `git commit` commits every changed file
automatically — this is the single most common Git surprise for beginners, and it's exactly
what Module 3's Lab will make concrete.

**Discussion question:** "Why would you ever WANT to stage only some of your changes, not all
of them?" (Answer: to build one commit about ONE logical change, even if your working tree
currently has several unrelated edits in progress.)