There is an ancient looking door in Saint-Michel-l'Observatoire that always has a pen and a sheet of paper in a frame attached to it. People sometimes write comments or messages on it.
<a href="//commons.wikimedia.org/wiki/User:Compo" title="User:Compo">Compo</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>