  We consider the problem of finding positive solutions of the problem $\Delta
u - \lambda u +u^5 = 0$ in a bounded, smooth domain $\Omega$ in $\mathbb{R}^3$,
under zero Neumann boundary conditions. Here $\lambda$ is a positive number. We
analyze the role of Green's function of $-\Delta +\lambda$ in the presence of
solutions exhibiting single bubbling behavior at one point of the domain when
$\lambda$ is regarded as a parameter. As a special case of our results, we find
and characterize a positive value $\lambda_*$ such that if
$\lambda-\lambda^*>0$ is sufficiently small, then this problem is solvable by a
solution $u_\lambda$ which blows-up by bubbling at a certain interior point of
$\Omega$ as $\lambda \downarrow \lambda_*$.
