  Any perturbation to a disc galaxy that creates a misalignment between the
planes of the inner and outer disc, will excite a slowly evolving bending wave
in the outer disc. The torque from the stiff inner disc drives a retrograde,
leading-spiral bending wave that grows in amplitude as it propagates outward
over a period of several Gyr. This behaviour creates warps that obey the rules
established from observations, and operates no matter what the original cause
of the misalignment between the inner and outer disc. The part of the disc left
behind by the outwardly propagating wave is brought into alignment with the
inner disc. Here we confirm that mild warps in simulations of disc galaxies can
be excited by shot noise in the halo, as was recently reported. We show that
the quadrupole component of the noise creates disc distortions most
effectively. Bending waves caused by shot noise in carefully constructed
equilibrium simulations of isolated galaxies are far too mild to be observable,
but perturbations from halo substructure and galaxy assembly must excite larger
amplitude bending waves in real galaxies.
