[Fluid Mechanics: Fundamentals] What are fluids?

A talk on what are fluids? why the materials which could flow are not fluids, while some materials flows extremely slowly which can be seen over a long time can be fluids? the answers for these questions are based on the physical understandings, including: Simple fluid definition vs. accurate fluid definition Simple experiment of flows (water, salt and flour) Unique fluid properties: pressure and viscosity Introduction of 4 pitch tar experiments Fluids vs. Solids: how to distinguish Proof: granular materials are not fluids