I have been reading a number of aerodynamic texts, and it seems that everything in a fluid (gas) is transmitted by molecular collisions. Pressure is a result of molecules colliding with each other and the walls of a container Viscosity is caused by collisions between molecules in layers of fluid with different velocity Thermal energy is "conducted" thru a gas by more energetic molecules colliding with less energetic molecules. Is there anything that is communicated by some other "transport" mechanism in a fluid besides collisions? I know that liquids have intermolecular forces because of how close the molecules are. Since there is only two ways that a fluid can communicate forces to a body immersed in it 1) Normal Forces (pressure) 2) Tangential Forces (shear - friction) In an "in-viscous " flow, you would still have fluid pressure caused by collisions, I assume that I could make the following argument: If I was to make a fluid element my "control volume" then all collisions in my control volume would be "pressure communicating collisions, only if a property such as energy or momentum is "transported" outside my control volume would I say that thermal conduction or viscosity has been enabled. Is this a proper analogy?