A glass over a drain, a scrap of paper, and the promise that you have sealed your house from sewer smell and bugs looks clever in a video. It is not a real seal. When you leave town, the water sitting in a P-trap can dry out. That water is the barrier between your pipes and the sewer. Once it is gone, odor and insects can travel up the drain. The fear is real. The glass-and-paper fix is not.
An upside-down glass almost never sits airtight on a sink. Paper soaks, warps, and lets air move. Heat, a fan, or a few extra days away finishes the job the video pretended to prevent. You board a plane thinking the drain is closed. The trap is still open to whatever is in the line.
The useful habit is simpler. Before you leave, run water in every sink, shower, and tub for ten to twenty seconds. Flush the toilets. Do the basement floor drain and the laundry standpipe too. Those forgotten openings dry out first. For a trip longer than a few weeks, use a real cover: a built-in stopper, a rubber or silicone drain cap, or plastic wrap taped tight. If you will be gone a long time, a tablespoon of mineral oil on top of a filled trap slows evaporation. Shutting off the main water supply is wiser still.
None of that looks like a life hack. It is just plumbing. The viral setup sells peace of mind with a balancing act that fails as soon as the paper gets wet. Fill the traps. Cover what you can. Leave the glass in the cupboard.
