Write one 41-byte file to branch with nothing copied, watch a switch rewrite every working file to another snapshot while your commits hold still, drive two branches into divergence, and see why `-d` refuses before it strands a commit.

You run `git branch dark-mode` and a second 41-byte file lands next to the first, holding the identical 40-character hash. Your whole project, every file, was left exactly where it sat, and it finished instantly.

Drive two branches to divergence and watch one reachability check pick the write: a reachable tip fast-forwards a 41-byte ref with no new commit, a diverged tip builds a merge commit with two `parent` lines, and standing on the other branch swaps which is first parent.

`git merge feature` sometimes writes a brand-new commit and sometimes writes nothing at all, just nudges a pointer. Same command, same two branches, two different results on disk, and one check decides which.

Halt a merge on one overlapping line, read the disagreement as marker blocks, three index stages, and a MERGE_HEAD flag, flip between merge, diff3, and zdiff3 rendering, then stage the fix into a two-parent commit or abort clean.

`git merge feature` prints a conflict and stops. Two teammates changed the same line of `config.json`, and git refuses to guess which one wins, so it hands the disagreement back, written out in plain text on disk.