Cause of Parallax?

[rockstar.esq]

First off, I know what it's like to experience parallax as well as how to correct it. What I don't fully comprehend is what is causing it. Focus makes sense because you're adjusting the distance between lenses which corresponds to the focal length. I'm guessing it has something to do with reticles because camera lenses don't have parallax adjustments (or if they do it's called something different). So if some kind THR's could help explain it, I'd be much obliged. Thanks in advance!

 

[rockstar.esq]

Just found a link answering my own question! Well in case anybody wanted to know more about it I'll post it here.

http://www.6mmbr.com/parallax.html

 

[MVF]

Thanks rockstar- good explanation