A toilet is a self-priming siphon pump with no moving parts in the waste path, built to move a fixed volume of water fast enough to trigger an air-vacuum effect it then has to break on purpose. The flush valve, fill valve, and trap geometry all exist to hit precise timing and volume targets — and the last thirty years of low-flow mandates turned that timing problem into the industry's hardest engineering fight.
Fluid-Dynamics
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How a Toilet Actually Works