These "messaging apps" are surpisingly useful. For example, you could hang them from your door handle as a "do not disturb" signal. Alternatively, you could flip it over and write messages on each side stating either, "Do Not Disturb" or "Visitors Welcome."
Other messages for other circumstances could include:
To make the lid, just extract the disc.
Of course, the dry erase board serves a similar function.
Similarly, I use a small, discreet aluminum strip of can around the temple arm of my glasses to distinguish my reading glasses from my identical distance glasses.
