Flows of particles in liquids occur in many geophysical and industrial problems, including sediment transport by wind and streams, pumping of slurries and suspensions, and landslides and debris flows.  How do the particles affect the transport properties, and what is the contribution of particle collisions? An influential study that addressed these questions was experimental and analytical research by R.A. Bagnold. His 1954 paper on the rheological properties of liquid-solid suspensions continues to be widely referenced because it purportedly demonstrated a transition from a viscous regime to a grain-inertia regime involving particle collisions. Work at Caltech, however, showed that transition observed by Bagnold was a result of secondary flows and fluid turbulence rather than particle collisions. In addition to revisiting the studies by Bagnold, this presentation overviews related experiments at Caltech on the rheology of liquid-solid flows, extending from viscous-dominated to turbulent flows, along with detailed studies of immersed particle collisions in liquids.