ZWICKY1 argued long ago that tidal forces can tear long tails of stars and gas from the bodies of interacting disk galaxies, and that this debris may include self-gravitating objects which could become small galaxies. Some recent observations revealing small clumps of stars and gas in tidal tails lend weight to this idea2-4. Here we report the results of numerical simulations of encounters between disk galaxies5, each modelled with a central bulge, an exponential disk and a spheroidal dark-matter halo. We find that dwarf systems form in material drawn out during the encounter; these objects can capture large amounts of moderately enriched gas, but retain little dark matter from their parents' haloes. They should therefore have lower mass-to-light ratios than galaxies formed directly by the collapse of primordial material.