Final answer:
Water flows down due to gravity. A fountain's water stream widens as it rises because of reduced pressure and gravity's effect, while a falling water stream narrows due to gravity and air pressure, enhanced by surface tension. The Coriolis effect influences water rotation in drains, but its effect is minimal at household scales.
Step-by-step explanation:
Water always flows toward the lowest point due to gravity. This is an unassailable truth in physics, representing a fundamental part of how fluids behave on Earth. When a container, like a flask, is tilted, the water molecules, which are in constant motion, will move in the direction that gravity pulls them, which is always towards the Earth's centre. The movement of the molecules to the lower side causes the water to flow out of the openings, creating a net flow out of the container. In the case of a decorative fountain, as water is shot upward, it experiences a downward force due to gravity, which slows down the upward motion. As the velocity of water decreases with height, the pressure also reduces, causing the stream to broaden. Conversely, as water falls from a faucet, it accelerates due to gravity, the surrounding air pressure compresses the stream, and it narrows. Surface tension serves to minimize the surface area and will enhance this narrowing effect for the falling water. When you flush a toilet or drain a sink, the water that flows straight towards the drain might begin to rotate due to the Coriolis effect, which is a result of the Earth's rotation. However, this has a very small effect on household scales, and often, the direction of water rotation is determined by the shape of the toilet or sink and the position of the jets. If water were forced upward through the drain, other forces would come into play, and the direction of rotation would depend on the direction of the applied force.