A <a href="https://en.wikipedia.org/wiki/citation_needed" class="extiw" title="w:citation needed">citation needed</a> sign suitable for use on signage denoting carrier's questioning of present subject matter, and the source of the knowledge that's arisen.
<a href="//commons.wikimedia.org/wiki/User:Hamiltonham~commonswiki" title="User:Hamiltonham~commonswiki">Hamiltonham</a>
<span class="int-own-work" lang="en">Own work</span>