We're going on vacation, and my in-laws told me to cover my sink drain with a piece of paper and place a glass upside down over it.

The packing is done, the lights are off, and you are standing at the front door ready to head out for a relaxing vacation. But right before you leave, a piece of classic, generational home advice stops you in your tracks: Cover your sink drains with a piece of paper, and flip a heavy glass upside down over the top.

It sounds like a quirky urban legend or a bizarre superstition, and it is completely normal to wonder what exactly this trick is supposed to accomplish. As it turns out, your in-laws are handing down a highly practical, old-school plumbing hack designed to prevent two major vacation nightmares: sewer gas leaks and pests.

1. The Pest Barrier: Stopping Hidden Intruders

The primary reason for this trick comes down to basic biology. When a house is occupied, water flows down the drains constantly, making it difficult for creepy-crawlies to make their way up into your living spaces.

However, when a home sits completely dark, quiet, and undisturbed for a week or two, your pipes become a peaceful highway.

• Roaches and Waterbugs: Certain pests, particularly American cockroaches (often called waterbugs), thrive in the dark, damp environment of sewer lines. When a home is empty, they will boldly climb up through the P-trap and enter your kitchen or bathroom.

• Drain Flies: These tiny pests lay eggs in the organic film inside your pipes. Covering the drain prevents adult flies from escaping into your home and breaking out into a full-blown infestation while you're away.

By placing a piece of paper (which forms a flat seal) and weighting it down with an upside-down glass, you create an impenetrable physical barricade. Any pest trying to crawl out will be completely trapped beneath the glass.

2. Preventing Dried-Out P-Traps and Sewer Gas

Every drain in your home features a U-shaped pipe underneath it called a P-trap. Under normal circumstances, a small amount of water always sits at the bottom of this "U." This water acts as a natural, airtight seal that prevents foul-smelling, dangerous sewer gases from traveling up the sewer line and entering your home;