TITLE: Why can't liquids oppose tangential forces acting on them?
QUESTION [1 upvotes]: Also, when a glass of water is inclined at a certain angle, why does the surface of water continue to remain parallel to ground?

REPLY [0 votes]: If it could oppose tangential forces indefinitely, then you wouldn't call it a liquid. The very definition of a liquid is a substance that deforms to fill up available horizontal space.
The viscosity of liquids does cause non-indefinite opposition against tangential forces, although not forever. Thick liquids like honey are still liquids.
But when you have "liquids" being so thick that they take ages to deform - or that only deform under certain conditions, such as under pressure - we tend to not call them liquids anymore although they also aren't solids (such as tooth-paste and wet beach sand).
{"set_name": "stack_exchange", "score": 1, "question_id": 440261}