That’s actually a useful vacation-home trick, although the paper-and-glass method isn’t foolproof.
The idea is to cover the drain while you’re away to help prevent sewer odors, insects, and debris from coming up through the drain. An upside-down glass can also serve as a visible reminder that the drain is covered.
A better way to do it
Before leaving:
Clean the sink and drain and remove food debris.
Run some water down the drain so the P-trap (the curved pipe underneath) is filled with water. This water barrier normally blocks sewer gases.
Place a drain stopper or tightly fitting cover over the drain. This is more reliable than a piece of paper.
If you’re leaving for a long time, cover the overflow opening too, if your sink has one.
Clean and dry the surrounding sink before you leave.
Why the glass?
The upside-down glass isn’t doing anything magical—it mainly holds the paper in place and creates a physical barrier over the drain. If the paper gets damp, however, it can stop being effective.
So your in-laws’ trick has a sensible purpose, but a proper drain stopper is the better solution.
