Credit the “stink trap,” which pretty much does what its name implies, stopping the stink from coming up from the sewer system.
It stops the stench of the sewer from rising up and bothering the inhabitants of your house.
How does the stink trap work? Oh, it’s a marvel of primitive technology, but it took humans a remarkably long time to figure it out.
In fact, President Chester Arthur reportedly fled the White House in 1882 in fear and revulsion when a sewer sent gas up through the sinks, baths, and toilets. If only he’d had stink traps.
The stink trap is that loop in the pipe that’s under your sink. In the old days, the pipe was shaped like an S, but nowadays, it’s shaped more like a P facing downward. Ironically; toilets still have an S-trap when a “P”-trap would be a much more appropriate name.
How does the loop shape keep the smell out? Water gets trapped in the lower loop of the pipe, creating a seal that doesn’t allow gases to pass either way.
That keeps sewer gases out of your house (and, for that matter, your house gases out of the sewers).
Alligators don’t live in sewers, that’s a myth. Goldfish won’t survive there either, no matter what your older brother might’ve told you before he flushed yours.