It may look like a strange superstition, but putting paper over a sink drain and weighing it down with an upside-down glass is meant to help block odors and small insects while a home sits empty. Under most sinks is a curved section of pipe called a P-trap. It normally holds a small amount of water, creating a seal that prevents sewer gases from traveling back through the drain and into the house.

When a sink isn’t used for an extended period, some of that trapped water can slowly evaporate. If the water level drops far enough, unpleasant sewer odors may enter the room. The paper and upside-down glass create an additional physical barrier over the drain. It may also reduce airflow around the opening and make it harder for drain flies or other small pests to emerge.

The trick isn’t foolproof, though. The most important step before leaving is simply running water through every rarely used drain so the traps are full. That includes bathroom sinks, showers, tubs, and floor drains—not just the kitchen sink. For homes left vacant for a long time, plumbers sometimes recommend additional methods to slow evaporation rather than relying solely on a glass and paper.

So your in-laws aren’t doing something completely random. The upside-down glass acts as a simple backup cover while nobody is home. It won’t fix clogged pipes or plumbing problems, but for a short vacation it can provide an extra barrier against the two surprises nobody wants to return to: a house smelling like the sewer or tiny insects coming through an unused drain.