Just started working in facilities maintenance, I deal with toilets a lot but don’t understand the magic completely.
In: Engineering
A toilet is fundamentally a bowl with a hole in the bottom, and a pipe coming from the hole which first curves upward a bit before going down to the sewer.
As long as the water level in the bowl is lower than the height of the upward-curving drain pipe, the water just sits in the bowl.
If you add a small amount of water or other stuff to the bowl after it reaches the height of the pipe, the extra water spills over the edge of the drain pipe into the sewer and the water level in the bowl remains constant.
However, if you add a large amount of water to the bowl at once, this completely fills up the drain pipe. This means that water falling into the sewer will leave behind vacuum rather than air, creating a siphon effect; all of the water in the bowl (and everything in the water) is sucked down into the sewer. This is called a “flush”.
You can flush a toilet bowl manually by pouring a bucket of water into the bowl, and then refill it manually by pouring some more water into the bowl.
However, the toilet tank is designed to automate both the flushing and refilling processes. When you pull the lever, the water in the tank is dumped into the bowl, causing a flush. Inside the tank is a float that detects water level in the tank; when it detects that the water level is low (i.e. that the toilet has just been flushed), the float activates a faucet that adds water to both the tank and the bowl, refilling them.
The diagrams on [this site](https://www.1tomplumber.com/toilet-flush-is-slow/) are fairly clear if you’d like to see a visual.
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