When you tilt a bottle, the water that flows into the cup primarily comes from the bottom of the bottle, not the surface. This is because liquids, like water, tend to flow downward due to gravity.
As you tilt the bottle, the force of gravity causes the water to move towards the lowest point in the bottle, which is usually the bottom. As a result, the water from the bottom of the bottle flows out first and fills the cup.
The water at the surface of the bottle may also flow into the cup, but its movement is usually minimal compared to the water from the bottom. This is because the surface water is not directly influenced by gravity in the same way as the water at the bottom.
It's worth noting that the specific behavior of the water flow can depend on various factors, including the shape of the bottle, the angle at which it is tilted, and the viscosity of the liquid. However, in most cases, the water that flows into the cup when you tilt a bottle predominantly comes from the bottom portion of the bottle.