How to play Critical Mass — Chain Reaction

Critical Mass is a chain-reaction territory game for two players. The rules fit in a paragraph and the consequences do not.

The board

Six columns by six rows, thirty-six cells. Every cell has a capacity equal to the number of cells directly touching it: four corners hold one orb before going critical, the sixteen edge cells hold two, and the sixteen middle cells hold three. That single rule is why the board is not uniform. The corners are the cheapest detonators in the game and the middle is the most expensive, and every position you build is shaped by that difference.

A turn

On your turn you tap one cell. It must be empty or already yours — you can never add to a cell your opponent owns. One orb goes in and the cell becomes yours if it was not already.

If that cell is now holding as many orbs as it has neighbours, it goes critical. It empties, and one orb travels into each neighbouring cell. Every cell that receives an orb becomes yours, regardless of who owned it before. That conversion is the entire point: you do not capture by counting or by surrounding, you capture by detonating next to something.

The chain

A neighbour that has just received an orb may itself be at capacity. If so it detonates as well, in the next ring, pushing into its own neighbours and converting them. This continues until no cell on the board is at or over its capacity.

The chain runs one ring at a time, and on a loaded board it can cross the whole grid. A single tap in the right place has been known to take twenty cells. The board does not gain or lose orbs while it does this — a detonation removes exactly as many orbs as it hands out — so the total number of orbs on the board is always the number of moves played.

Winning

You win when your opponent has no orbs left on the board at all.

The opening is graced. Left as written, that rule makes the first few moves a coin toss — a corner detonated on the third move wipes an opponent who owns one cell, and the match is over before either side has a position. So the wipe rule only goes live once both players have had four turns, which is move eight. Nothing is frozen in the meantime: a player holding no orbs can always place on an empty cell, so the grace is a real second chance rather than a pause. From move eight onward a wipe ends the match immediately, mid-chain if that is where it happens.

Why the chain always stops

An unbounded chain reaction is a game that freezes, and this is not a hypothetical: on a heavily loaded board a naive implementation genuinely never settles. Critical Mass handles it with an argument rather than a limit.

A cascade never changes the number of orbs on the board, and the number of orbs equals the number of moves played. Chip-firing theory says a board holding fewer chips than the grid has edges must always settle — the six-by-six grid has sixty edges, so every cascade in the first fifty-nine moves of a match is guaranteed to stop by mathematics alone, with no guard involved. Matches essentially always finish well inside that window.

Beyond it, the stop is a rule of the game rather than a cap. Before each ring the engine asks whether the mover now owns every orb on the board; if they do, the match is already decided and the chain is cut. That is guaranteed to happen on any chain that would otherwise be infinite, because an infinite chain forces every cell on the board to detonate, and every cell that detonates or is detonated into belongs to the mover.

Playing by link

A match is nothing but its list of moves, and with thirty-six cells one character encodes one move. A sixty-move match is therefore about eighty characters of URL, and opening that link replays the match exactly — the last move plays out in full so you see what just happened to you — and hands you the next turn.