  It has been known for a long time that a large fraction of disc galaxies are
lopsided. We simulate three different mechanisms that can induce lopsidedness:
flyby interactions, gas accretion from cosmological filaments and ram pressure
from the intergalactic medium. Comparing the morphologies, HI spectrum,
kinematics and m=1 Fourier components, we find that all of these mechanisms can
induce lopsidedness in galaxies, although in different degrees and with
observable consequences. The timescale over which lopsidedness persists
suggests that flybys can contribute to ~20 per cent of lopsided galaxies. We
focus our detailed comparison on the case of NGC 891, a lopsided, edge-on
galaxy with a nearby companion (UGC 1807). We find that the main properties of
NGC 891 (morphology, HI spectrum, rotation curve, existence of a gaseous
filament pointing towards UGC 1807) favour a flyby event for the origin of
lopsidedness in this galaxy.
