Changes

The trap is very simple. Imagine a capital "P" turned on its side so the fat part points down. As you run water through the sink, wastewater will flow down into the "P" and back up down the stem into the DWV riser. When you shut off the sink, a small amount of water remains trapped in the "P," blocking the gasses. The water does not get rancid because you change it every time you run the faucet.