Multiphase domain needs exactly two materials. Incorrect number of material assignments for the following entities

Hi again, thank you very much for your thorough answer. I still have a of question I hope you could clarify:

You assigned the materials air and water to every part of the solid. I guess that makes sense because every part of the solid can potentially contain water or air? When I originally assigned air to the two top smaller solids and water to the bottom solid, I basically said that air could never leave the small solids and likewise for water?

Thank you