How to explain Post Modernism to Mrs B

I've always had some difficulty with post modernism. Nothing I had previously looked or thought about, or conceived to be 'art' could help me to understand/appreciate/explain/comment on post modernist works. I'm clutching at straws, jumping in the pool at the deep end. I have struggled to comprehend or feel the line where art history stops and borders on/transitions into art philosophy.
Yesterday I feel I have finally grasped the way to go ahead and see things differently (with my eyes, with my thoughts). It's about letting go of all things you've ever known or used to look at art. But I think I finally grasp the Art after Art theory.
It is difficult to view PM art. One artist wants us to look at his work and feel things, another wants us to reflect upon the products of his imagination.
Some works make us feel alienated, force us to let go of presumptions, challenge our preconceptions about art or even life. Some artists shock for the sake of shocking. Some artists don't want to make self explanatory works or explain their works, according to some explaining means missing the point of making (or defeating the purpose) of art. Some concoct theories, implement them in their work, and then make some works that do not totally abhor to their original theory. The same goes for art critics or connoisseurs.

Art has always been elitist and inaccessible to most people. I would have to make an exception for religious and ethnic art though.
Well, at least everyone will look at it with some level of understanding. But the prime feature is that it has a clear purpose. And granted, art has in some way, shape or form been masquerading as sheer propaganda (think Contra-Reformation and Baroque). But never has there been such debate on art as in the last sixty years.
First thing that opened my eyes was to view PM in a historical context. I had done so previously with the periods covering 'traditional' art up to the apparition of expressionism. Most things exhibited from later periods in musea I would glance at them sideways or walk right by.
It is so simple to do the same for PM. The time scale is much more condensed than the previous millennia. But society has also changed very dramatically in sixty years time. A work made in the seventies would not be the same in the nineties. (I have to think of the question 'why wasn't this work made 100 years earlier?').

So why the big "Eureka" moment? I've been reading up on Joseph Beuys for my Contemporary Art class. Beuys is different.
He wasn't at first to me. Not in the Art History Classes I took ten years ago. He is now. Now that I've made an effort to delve deeper into his works, void of all the commentaries of everyone who wants to voice or has voiced an opinion about him or his works.

If we look at an artist like Beuys, people today still say 'Its not art', 'He is sheer genius', 'These are the ramblings of a crazed loon',...
Are these statements the consequence of not understanding his work or trying to envelope oneself in the 'myth' of the elitist crowd hailing this as the best thing ever? Who dictates if something is art? Why is it considered art now and was it hailed as the next best thing/booed all those decades ago?

In his work, at the time, he was not the only person doing performances. Beuys was different. Very much so in his approach to art and education. After an eight hour performance he would still interact with the audience and explain what it was all about.
I must say his work is very accessible. This might seem a bold statement, but it makes more sense to me now.
If people hear the word 'art' or 'art education' I don't think the first thing they think of will be a Beuys performance. "Wie man dem toten Hase die Bilder erklärt" It does seem goofy to go around with a dead hare in your arms, your face covered in gold leaf, explaining paintings to it.
But Beuys has opened my eyes on how to look at PM art.