system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When you rotate a glass of water, why does the ice stay in its original position?

I assume this is a physics question?
gpt
The ice cube isn’t moving because you are actually transferring very little rotational momentum to the ice cube. 

There is a tiny layer of water between the ice cube and glass that lubes the surface so that there is very little friction. 

Remove the water from the equation and that layer of water exists to a much lesser extent if at all. So then the friction between the glass and ice cube is sufficient enough to transfer the rotational momentum to the ice cube and so it moves.