Critical Mass — Chain Reaction

About this game

Critical Mass is a two-player chain-reaction game on a six-by-six grid, and it is the only game in this catalogue you cannot play alone. Every cell holds one fewer orb than it has orthogonal neighbours, so a corner goes off on its second orb, an edge cell on its third and a middle cell on its fourth. Tap a cell you own or an empty one and you add an orb to it. Fill a cell to its neighbour count and it detonates: it empties, pushes one orb into every neighbour, and every cell it touches becomes yours. Those neighbours can hit their own limit and go off in turn, so a single tap can roll across the board in a widening front and hand you territory you never touched. You win when your opponent has no orbs left anywhere. The whole game is the tension between loading a cell and being ready for what happens when it goes. A cell one orb short of critical shivers on the board — that tremor is the only warning either player gets, and it is a warning to both of you, because whoever taps a neighbour next takes the whole chain. Corners are cheap detonators and the middle of the board is a slow fuse, so the geometry matters as much as the count. There are three ways in. Pass and Play is the game as it is meant to be played: two people, one phone, taking turns. Solo faces you off against one of three opponents that differ in what they can see: Casual counts orbs and takes what it finds, Steady adds the board's shape and plays one move deep, and Ruthless searches your best answer before it commits to anything. Start on Casual — a first-time player takes a game off it roughly one time in four, while Ruthless will not lose one. And a match can be played entirely by link — the whole game encodes as one character per move, so a sixty-move match travels in about eighty characters of URL. You make one move, send the link, they open it and watch your move land before they take theirs. Nothing in the game is random, so a link replays a match exactly. Every chain in Critical Mass is proven to settle. A chain reaction with no bound is a hang, and on a loaded board a naive version genuinely never stops — so the engine's guarantee is stated as an argument and checked by a prover, not capped and hoped for.

Controls

Tap or click a cell to drop one orb into it. You may only use an empty cell or a cell you already own.

Touch and mouse

• Tap an empty cell or one of your own to add an orb; the cell turns your colour.
• A cell one orb from critical shivers — tapping it detonates it into every neighbour.
• Tapping a cell your opponent owns does nothing and flashes red.

How to play

Two players take turns adding orbs. Every cell can hold one fewer orb than it has orthogonal neighbours, and filling it past that sets it off.

Capacity

• A corner cell touches two neighbours, so it detonates on its second orb.
• An edge cell touches three and detonates on its third.
• A middle cell touches four and detonates on its fourth, which makes it the slowest fuse on the board.

Detonation

• A detonating cell empties completely and pushes exactly one orb into each of its neighbours.
• Every cell it pushes into becomes yours, whoever owned it a moment ago.
• If a neighbour is now at its own capacity it detonates too, in the next ring — that is the chain, and it is the whole game.

Winning

• You win the moment your opponent holds no orbs anywhere on the board.
• The first four turns each are a grace: a wipe inside them does not end the match, and a player on zero orbs can always play an empty cell.
• From move eight onward the wipe rule is live, so a chain that clears their last orb ends it there and then.

Modes

• Pass and Play hands the phone back and forth between two people.
• Casual, Steady and Ruthless are the three solo opponents, in that order of strength, and only Ruthless looks at your reply before committing.
• Challenge by link encodes the whole match in the URL, so you can play a move a day with someone anywhere.

Read the full Critical Mass — Chain Reaction rules and strategy →