How Git keeps every version of your project without filling your disk

Make a new branch in Git, even in a project of a million lines, and not one file is copied. Undo one step too many, and days of commits drop out of your history: are they gone? The answers to both come from two simple ideas inside Git, and you build them up with a small frog game made by two friends: commits and snapshots, the staging area, hashes and history, branches and HEAD, fast-forward and merge commits, merge conflicts, clone, push and pull, reset and revert, and the reflog.