Don Vandervort, Head Homeboy, has written more than 30 DIY home improvement books, been a segment host on HGTV, served as MSN.com's home improvement expert and written countless magazine articles.
A sink trap is a J-shaped pipe beneath a sink, designed to catch noxious sewer gasses before they enter the house. The most common trap is sometimes called a P-trap (because it's shaped like a P that has fallen face down).
When water runs down a sink drain, it moves through the trap and exits out the drainpipe. But because the drainpipe exits at a higher level than the curved portion of the pipe, some water is captured and held in the trap's curve. This water provides a seal, blocking sewer gasses from rising up through the sink drain. Every time you use the sink, the water in the trap is replaced.
The trap is connected to the sink drain's tailpiece and the drainpipe with threaded couplings. A large rubber washer beneath each coupling provides a watertight seal.
If a sink becomes clogged--or if you drop a small object down the drain--the first place to look for the clog or the lost object is in the easily removed trap (be sure to put a bucket underneath before disassembling the pipes). Some traps have a clean-out plug at the bottom so you don't have to remove the entire trap for a clog.
