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:

  • Clean/Dirty
  • Privacy Please / Waiter Needed
  • Order #54
  • Claim Check #27
  • Mail/No Mail
  • Dog Fed/Not-Fed
  • Newest/Oldest
  • Last Cleaned (Maintained) on: 1/5/17

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.