  We study a variant of the chip-firing game called the diffusion game. In the
diffusion game, we begin with some integer labelling of the vertices of a
graph, interpreted as a number of chips on each vertex, and then for each
subsequent step every vertex simultaneously fires a chip to each neighbour with
fewer chips. In general, this could result in negative vertex labels. Long and
Narayanan asked whether there exists an $f(n)$ for each $n$, such that whenever
we have a graph on $n$ vertices and an initial allocation with at least $f(n)$
chips on each vertex, then the number of chips on each vertex will remain
non-negative. We answer their question in the affirmative, showing further that
$f(n)=n-2$ is the best possible bound. We also consider the existence of a
similar bound $g(d)$ for each $d$, where $d$ is the maximum degree of the
graph.
