system
Given the following post, respond as if you're commenting on Reddit:
human
When I pour water from a container with a small opening, why does it appear to twist?

I say water, but I suppose this applies to any liquid with low viscosity.  As the liquid leaves the exit, it seems to spiral down into the cup I'm pouring it in.  Why?
gpt
It's not twisting - it's oscillating.

Surface tension tries to pull a poured liquid into a circular cross-section.  If the liquid emerges from the spout with some other shape (say, an ellipse with the north-south axis being larger than the east-west axis), that bit of liquid will oscillate as it falls.  It will shrink along the north-south axis, and grow along the east-west axis, and inertia will cause it to overshoot.  Viscosity damps this oscillation.

As the liquid falls, it gathers velocity, and so its cross-sectional area diminishes.  At some point, the stream becomes so narrow that surface tension dominates and the amplitude of these 'elliptical' oscillations diminishes (while their frequency increases).  The stream becomes a narrow circle.  Eventually it stretches to the point that surface tension is insufficient to hold it together, and it breaks into droplets.