We present a new mechanism to explain the frequently observed and thus certainly permanent warping of spiral galaxies. We consider the possibility of non-linear coupling between the spiral wave of the galaxy and two warp waves, such that the former, which is linearly unstable and extracts energy and angular momentum from the inner regions of the galactic disk, can continuously feed the latter. We derive an expression for the coupling coefficient in the WKB approximation. We show that the coupling is too weak in the stellar disk, except at the Outer Lindblad Resonance where the spiral slows down and is efficiently coupled to warp waves. There, the spiral can be almost completely converted into ``transmitted'' warps, which we can observe in HI, and a ``reflected'' one, which we can observe as a corrugation. Our mechanism reproduces the observed amplitudes of the warp and of the corrugation, and might explain related phenomena such as the behavior of the line of nodes of the warp. Furthermore we show that the energy and momentum fluxes of observed spirals and warps are of the same order of magnitude, adding a strong point in favor of this model.