  We provide a model for the remarkable stability of surface nanobubbles to
bulk dissolution. The key to the solution is that the gas in a nanobubble is of
Knudsen type. This leads to the generation of a bulk liquid flow which
effectively forces the diffusive gas to remain local. Our model predicts the
presence of a vertical water jet immediately above a nanobubble, with an
estimated speed of $\sim3.3\,\mathrm{m/s}$, in good agreement with our
experimental atomic force microscopy measurement of $\sim2.7\,\mathrm{m/s}$. In
addition, our model also predicts an upper bound for the size of nanobubbles,
which is consistent with the available experimental data.
