The trap under your sinks and under your tub and shower.
I get many questions about the 'U' or "J" shaped
fitting under the sink.
Many people think a plumbing vent is to vent the sewer odor from the house, but that is not it's function, a vent pipe brings air into the drainage system to allow the water to drain, next time you have a glass of something that you use a straw to drink,
If You hold your finger over the top of the straw, note how whatever is in the straw will stay in the straw even if you lift it all the way out of the glass, but as soon as you remove your finger from the top of the straw the liquid drains right out of the straw, and that is what the drain vents do in your drainage and vent system.
are a S trap themselves, and you can see the
siphon action take place when the toilet
flushes, but the toilet is designed to refill
the bowl after every flush, keeping the trap
full of water, the toilet also has a vent that
goes through the roof, a toilet can not flush
without a vent.