It has to be the Double-Slit Experiment.
<a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=DfPeprQ7oGc" rel="nofollow">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=DfPeprQ7oGc</a><p>I still remember how my mind was blown when I first saw it.