If you turn a cup filled with water with a few pieces of ice, the ice does not appear to move. Why?
In: Physics
Objects at rest remain at rest, unless acted on by an external force.
If the spinning glass isn’t touching the ice, the only way for it to transmit force to the ice is to first spin up the water. Since the glass is usually circular, the water is not forced to deform as the glass spins, and the only interaction is the slight friction between the water and the glass’ walls—essentially a simple [fluid bearing](https://en.wikipedia.org/wiki/Fluid_bearing). With very little friction between the glass and the ice, it takes a lot of glass-spinning to get the ice moving noticeably.
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