The word sewer has two completely unrelated meanings:

1. An underground pipe or tunnel that carries away wastewater and sewage from buildings. When you flush a toilet or water goes down a drain, it flows through pipes into the sewer system beneath your street. These tunnels carry waste to treatment plants where it's cleaned before returning to rivers or oceans. In cities like Paris and London, some sewers are so large that workers can walk through them for inspections and repairs. Before modern sewers, cities dumped waste in streets or rivers, causing terrible diseases. The invention of sewer systems in the 1800s was a breakthrough in public health, making cities much safer places to live.
2. Someone who sews, pronounced differently (like “SO-er”). A skilled sewer might make clothes, repair torn backpacks, or create quilts. Your grandmother might be an excellent sewer if she can stitch a Halloween costume from scratch.

The two meanings sound different when spoken: the underground tunnel rhymes with “fewer,” while the person who sews rhymes with “mower.” In writing, context makes the meaning clear. You wouldn't confuse someone stitching a button with an underground waste tunnel.