Siphon
Definition
Draining a tank over its rim with a bent tube — once started, gravity and air pressure keep it flowing, uphill first, then down.
Technical definition
Flow driven by the hydrostatic pressure difference between the ends; lift limited by atmospheric pressure to ~10 m. Liquid cohesion maintains the column.
Example
Aquarium owners siphon water for cleaning — one pull to start it, then gravity does the work until the tank level drops to the tube.
Common confusion
A siphon is not perpetual motion — it only flows downhill overall. The 'uphill' leg is paid for by a longer downhill drop on the other side.