Goal: take over the board. You win when your opponent has no atoms left.
Take turns adding one atom to a cell: an empty cell, or one that's already yours.
Each cell can only hold so many atoms: a corner holds 1, an edge cell 2 and a middle cell 3. One more than that and it bursts.
A burst sends one atom into each neighbouring cell, and those cells become your colour, atoms and all. If that overfills them, they burst too, so one move can set off a chain across the board.
Atoms that shake are one away from bursting: watch out for your opponent's, and use your own.
The win is only checked once both players have had a turn.
Controls
Mouse or touch: tap a cell.
Keyboard: arrow keys move · Enter or Space places.