Sudoku.

Three rules. That's the whole game.

When only one digit fits.

Pick a digit. Find where it has to go.

The tool that holds your thinking.

When a box tells a row where a digit can't go.

The Pointing Pair's mirror image.

Two cells, same two candidates, both spoken for.

Two digits with nowhere else to live.

Three cells, three candidates, locked.

Three digits with only three cells left to live in.

A digit caught in a rectangle.

Three cells. Three pairs. A chain of forced moves.

X-Wing scaled to three rows and three columns.

Two rows sharing a base, two different tops.

Two bivalue cells, joined by a strong link.

An L-shape in a box, finishing a chain.

Two colors, a chain of strong links, an elimination.

Two chains, one digit, four color groups.

Strong, weak, strong, weak. The framework.

Try both. If both paths force the same answer, lock it.