My sister left for vacation and asked me to place an upside-down glass over her kitchen sink drain before I left. She said it was important but never explained why. I did it but now I’m curious: what was it for?

We all have that one family member with a list of specific, quirky household rules. For me, it’s my sister. Before she headed out for a relaxing two-week summer vacation, she asked me to look after her place. Everything seemed standard—water the plants, collect the mail, ensure the doors are locked.

But right before I walked out the door on the final day, I remembered her last, most unusual request: “Please place an upside-down drinking glass over the kitchen sink drain.”

She insisted it was incredibly important but rushed out before explaining the reasoning. Being a good sibling, I did exactly as she asked. But the moment I got home, curiosity got the better of me. Why on earth would an empty glass over a drain matter so much to a house left empty?

After doing a little digging and speaking with a few home maintenance experts, it turns out my sister is a genius. Here is the fascinating reason behind this old-school household trick.

The Main Reason: Creepy Crawlies are Looking for an Invite

When a house sits empty and quiet for a week or two, it becomes the perfect environment for unexpected guests. The biggest reason people place an upside-down glass over their sink drains is to prevent pests from entering the home.

• The Plumbing Highway: Main sewer lines and drain pipes are dark, damp, and full of organic matter—the exact environment where bugs thrive.

• The Empty House Effect: When you are home, the constant flow of running water, soap, and noise keeps critters from climbing up your pipes. But when a house goes dead silent and the pipes stay dry for days, pests like cockroaches and drain flies start exploring upward.

• The Perfect Barrier: An upside-down glass creates a heavy, solid seal over the drain opening. If any adventurous bugs manage to climb up the pipe, they hit an unbreakable wall and are forced to turn right back around.

Reason 2: Stopping the “Dry Trap” Odor

There is another brilliant reason for the glass trick that has to do with your home’s plumbing design.

Underneath every sink is a U-shaped pipe called a P-trap. This pipe is designed to hold a small amount of standing water at all times. That water acts as a natural seal to keep foul sewer gases from rising up the pipes and entering your kitchen.

However, during a hot summer vacation, or if a house is left for an extended period, that standing water can slowly evaporate. Once the water barrier is gone, those unpleasant odors can waft right into the kitchen, leaving the house smelling terrible when you return.

By placing a glass flat over the drain, you drastically slow down the evaporation process and keep any stray odors trapped firmly inside the pipes.

Other Brilliant Vacation Drain Tricks

It turns out that seasoned homeowners and grandmothers have been using variations of this trick for generations. If you don’t want to leave your favorite glassware in the sink, here are a few other popular methods to protect your drains while you are away:

The Rubber Mat Method: Plop a heavy rubber flat drain stopper or a coaster over the opening to seal it completely.

The Baking Soda Refresh: Before you leave, pour a 1/2 cup of baking soda down the drain followed by a cup of white vinegar. Flush it with hot water to clean out any food residue that might attract fruit flies while you are gone.

The Mineral Oil Trick: For very long trips, some homeowners pour a tablespoon of mineral oil down the drain. The oil floats on top of the water in the P-trap, completely stopping it from evaporating.

The Verdict

As it turns out, my sister wasn’t just being paranoid. Leaving an upside-down glass over the sink drain is a brilliant, zero-cost way to ensure you return to a clean, fresh, and bug-free home.

Next time you pack your bags for a getaway, you might just want to add this simple trick to your departure checklist!