  We address a simple model where the Kennicutt-Schmidt (KS) relation between
the macroscopic densities of star-formation rate (SFR, $\rho_{\rm sfr}$) and
gas ($n$) in galactic discs emerges from self-regulation of the SFR via
supernova feedback. It arises from the physics of supernova bubbles,
insensitive to the microscopic SFR recipe and not explicitly dependent on
gravity. The key is that the filling factor of SFR-suppressed supernova bubbles
self-regulates to a constant, $f\sim 0.5$. Expressing the bubble fading radius
and time in terms of $n$, the filling factor is $f \propto S\,n^{-s}$ with
$s\sim 1.5$, where $S$ is the supernova rate density. A constant $f$ thus
refers to $\rho_{\rm sfr} \propto n^{1.5}$, with a density-independent SFR
efficiency per free-fall time $\sim 0.01$. The self-regulation to $f \sim 0.5$
and the convergence to a KS relation independent of the local SFR recipe are
demonstrated in cosmological and isolated-galaxy simulations using different
codes and recipes. In parallel, the spherical analysis of bubble evolution is
generalized to clustered supernovae, analytically and via simulations, yielding
$s \simeq 1.5 \pm 0.5$. An analysis of photo-ionized bubbles about
pre-supernova stars yields a range of KS slopes but the KS relation is
dominated by the supernova bubbles. Superbubble blowouts may lead to an
alternative self-regulation by outflows and recycling. While the model is
over-simplified, its simplicity and validity in the simulations may argue that
it captures the origin of the KS relation.
