Maybe it's like trying to explain to Boromir in simple terms why he can't take the ring back to Gondor.
All Thevenin's and Norton's theorems amount to is that, for a linear circuit, the relationship between the voltage across any two points in the circuit and the current through those same two points (those two points are the "terminals") has a linear relationship that can be written as: Vx = Vth - Ix*Req This is the "load line" for the circuit between those two terminals. But this is exactly the same relationship that you get if you replace the circuit with a voltage source with an output of Vth in series with a resistance of value Req -- so these two circuits are equivalent as far as what happens between the two terminals.
If you're like me and you like to hear it "how it is" then pick up an industry standard reference book and memorise it word for word. That's what I do. But be warned, these books are only for the brave; not even a library will let you loan a reference book out. So I had to buy mine. I've always got a piece of paper squashed in my back pocket full of writing to memorise that I copied. My ambition? So long as I've got a chapter in a reference book with my name on it I'm happy.