By far my favorite explanation of it all, starts simple and explores it very well: <a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=YX40hbAHx3s" rel="nofollow">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=YX40hbAHx3s</a>