mass is what a tile weighs: the sum of its 128 cell weights, from 0 to 1,920. struck cells weigh nothing. mass rises when buys land on or near a tile and falls when sells do, so where a tile sits in the field matters as much as who holds it.

a buy thickens up to fifteen cells of the tile it presses on, one step each. then the growth rule runs across that tile and its eight neighbours: a cell lighter than the average around it gains a step, and a cell far heavier than its surroundings loses one. a sell does the reverse. it thins cells, and the decay rule pulls every cell that stands above its surroundings down a step. a tile surrounded by heavy tiles keeps its weight. a tile next to a run of sells loses it.