A Hacker News post explores fundamental questions about fluid mechanics through detailed thought experiments. The author examines why water remains static in a tank despite pressure gradients, noting that upward pressure force exactly counterbalances gravity. This observation leads to deeper questions about how fluids accelerate through pipes and spouts.

The piece investigates scenarios like water flowing through a horizontal pipe connected to a tank, questioning where acceleration occurs and how conservation of mass affects flow velocity. The author grapples with apparent paradoxes, such as why water speed depends on tank height but shouldn't accelerate inside a pipe due to incompressibility constraints. Various pipe geometries are considered to understand velocity distribution.

Through diagrams and reasoning, the exploration reveals that constant velocity across flow-orthogonal slices is a special case, not a general rule. The author concludes that real fluid behavior involves complex patterns including vortices and varying cross-sections. This self-directed investigation demonstrates how working through physical problems from first principles can reveal subtle insights about fluid dynamics, even for experienced practitioners.