A siphon, not a plug
Why Does a Toilet Flush by Itself?
Pushing the handle dumps water fast enough to fill a bent pipe, and a full bent pipe empties the bowl on its own.
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Step 1 of 5
The bowl is always half full on purpose.
Behind the bowl the pipe rises and falls again, and water sits in that bend permanently. It is a plug of water, and it is what stops drain smells coming back up.
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Step 2 of 5
The handle dumps the whole tank at once.
Speed is the whole trick. A trickle would drain away as fast as it arrived, but several litres in a few seconds fill the bend faster than it can empty.
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Step 3 of 5
A completely full bend becomes a siphon.
Once the bend has no air left in it, the water falling down the far side hangs on to the water behind it and drags it over the top.
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Step 4 of 5
It stops the moment air gets in.
The gurgle is the siphon letting go. As the bowl empties, the level drops below the mouth of the bend. Air breaks the chain of water, the pull vanishes, and you hear the gurgle.
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Step 5 of 5
Then it quietly rebuilds itself.
A float rides up with the refilling tank and shuts the inlet at the right level, while a thin side tube dribbles enough water back to reseal the bend.
A modern cistern holds about 6 litres. Toilets built before the 1990s used 13 or more for exactly the same job.
The short version
A toilet flushes itself because a sudden rush of water fills a hidden bend, turning it into a siphon that drains the bowl until air breaks the chain.
Try it yourself
Tip a bucket of water into the bowl quickly. It flushes without touching the handle — the siphon only ever needed the sudden rush.
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