  In this paper we fill in a fundamental gap in the extremal bootstrap
percolation literature, by providing the first proof of the fact that for all
$d \geq 1$, the size of the smallest percolating sets in $d$-neighbour
bootstrap percolation on $[n]^d$, the $d$-dimensional grid of size $n$, is
$n^{d-1}$. Additionally, we prove that such sets percolate in time at most $c_d
n^2$, for some constant $c_d >0 $ depending on $d$ only.
