Siphon
Fluid flows upward over a barrier and down the other side, driven by gravity and atmospheric pressure. No pump is needed — just a filled tube connecting two containers at different heights. The siphon breaks when the tube rises above ~10.3 meters, the height atmospheric pressure can support.
A siphon works by creating a continuous chain of fluid from the upper container, over the apex of the tube, and down to the lower container. Once primed (the tube is filled with fluid), gravity pulls the heavier column of fluid on the output side downward, which reduces pressure at the apex and draws fluid up from the input side.
The flow velocity is governed by Torricelli's law (a consequence of Bernoulli's equation): v = √(2gΔh), where Δh is the height difference between the two fluid surfaces. The volumetric flow rate is Q = A · v, where A is the cross-sectional area of the tube.
The siphon has a fundamental limit: atmospheric pressure can only support a column of water about 10.3 meters tall (at standard pressure, 101.325 kPa). If the apex of the tube rises above this height relative to the upper fluid surface, the pressure at the apex drops to zero (vacuum), the fluid column breaks, and the siphon fails. On Mars, with atmospheric pressure of about 0.6 kPa, a water siphon could only reach about 6 centimeters.
Viscosity slows the flow through friction against the tube walls. The Hagen-Poiseuille correction reduces flow rate for viscous fluids or narrow tubes. Honey (viscosity ~2000 mPa·s) siphons much more slowly than water (1 mPa·s).