system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: For example, we have a bottle filled with water to the point when there’s no space left in that bottle, is the water still moving as we shake the bottle?
gpt
Mechanical engineering PhD here with significant study in fluid mechanics. 

Yes the fluid will move around internally, mostly due to any rotation of the bottle. It you rotate the bottle at all (i.e. anything other than perfect linear motion) the inner walls of the bottle will cause drag against the water that’s closest to the edge. But the water in the middle of the bottle will be stationary due to inertia. The difference in momentum (edges are moving, center is stationary) will cause internal currents.

Note that this can be due to even very small-scale rotations to the point that this will always happen in the real world.

If you don’t believe me... put some food dye in a full water bottle then seal it. Shake it up and you’ll see the food coloring move around very quickly. 

If you’re more interested in this, two important concepts are “Laminar flow” and “viscous drag”.