system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does water mostly stay still while you turn a glass?

If you have a glass of water with ice and you try to turn the glass around to drink from another side, the ice will mostly remain in the same spot while the glass turns. So if the ice was hurting your teeth and you wanted to drink from an end where the ice isn't, it's not as easy as I feel it should be. What is happening and why is it happening?
gpt
Physicist here. 
So imagine that the water is made of several layers starting from the walls of the glass and going in. 
When you turn the glass, the layer of water that is touching the walls of the glass will rotate with the glass itself, because it is being directly pushed. Then, the first layer of water tries to push the second one (which is closer to the center of the glass), but since water behaves like a liquid there is less friction between the two layers of water than between the first layer and the glass. This way, the second layer won't rotate as fast. 
Add this up for all the next layers and soon the effect of the rotating glass will no longer be noticeable in the inner layers. 
To solve your problem, simply keep spinning the glass so more momentum is transfered over time to the inner layers, eventually making the ice in the center spin. 
Enjoy your drink!