The Cup of Water Under Your Sink That Blocks Sewer Gas 🚰

A cup of water hidden under your sink is the only thing stopping toxic sewer gas from filling your home.

Why is the pipe under every sink bent into a pointless-looking U instead of just dropping straight down?

That U-bend is a P-trap, and the bend is the whole point. Every time you run the tap, water flows through and settles in the bottom of the curve, leaving a small pool trapped there. That pool forms a seal: a plug that liquid passes through but gas cannot. Below it, your drain connects to the sewer and its foul, even flammable, gases. Those gases rise up the pipe, hit your pool of water, and stop dead. The waste goes down, but the smell can't come back up.

So the only barrier between you and the sewer is a few inches of water. In a drain you never use, that water slowly evaporates, the seal breaks, and the room just starts to stink.

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