Newton’s First Law of Motion. Paraphrased, it is “Objects in motion will remain in motion and object at rest will remain at rest unless acted upon by an unbalanced force.
This means that when you turn a cup the force of that cup needs to be transferred to the fluid in some way to make it move as well, otherwise it won’t. Except of course a classic feature of a fluid is that it flows which means the moving inner cup wall isn’t able to effectively transmit motion to the non-turning fluid, so the bulk of the fluid doesn’t turn right away.
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