If you’re stuck at home in the middle of a water (or power) outage, there’s a good chance you’ll end up reaching for the biggest bowl, pot, or bucket you can find. It’s a relatively open secret that dumping a large quantity of water in the toilet will trigger an automatic flush. Here’s how that practical magic works.
For most toilets, the handle is attached to a chain that’s attached to the flapper—which is basically the lid on top of the flush valve. As soon as you press the handle down, the chain lifts the flapper and the water from the tank flows through the valve into the toilet bowl. The hole in the bottom of the bowl connects to a tube called a siphon, which curves up above the resting water line of your toilet bowl before snaking sharply down. When the bowl gets rapidly flooded with water, everything’s forced through the siphon until there’s no longer enough water to fill the whole tube. There isn’t enough pressure to force the (new, clean) leftover water up over the tube’s highest point, so it settles back in the bowl.
As HowStuffWorks explains, this is why pouring a cup or two of water in your toilet bowl won’t generate a flush—it’s not enough to fill the entire siphon and therefore cause the contents of your toilet to get sucked down. A couple gallons, on the other hand, will do the trick. In short, by pouring a bunch of water in the toilet bowl, you’re essentially doing the toilet tank’s job for it.