I'm sure some won't agree with this explanation. For me it explained the basics of thrust.
It's 1969, I'm in my eighth grade Science class.
I thought I knew everything about model rocketry. I'd been building them for a year and probably had a few dozen flights under my belt.
I was talking to my science teacher about my new hobby.
He asked: "What makes your rockets move?"
I tried to explain: "The thrust gasses push against the air outside the rocket. That friction makes the rocket go up and fly."
He said: "No, no exactly." He added something like: "The model moves because the pressure inside the engine isn't even."
What? I still didn't get it. (That catalog drawing with all the arrows inside the cutaway engine didn't make sense.)
He grabbed a piece of paper and pencil and drew some stick figures in a box.
The box has no bottom, their feet are on the floor under the box sides.
(I'll try to explain what he showed me.)
"Here's Jim and Fred in a big cardboard box. They have a bet who will break through the box first. Jim is on the left, Fred is on the right. Both are pushing hard on opposite walls of the box.
First to break through their side wall wins."
"Fred (on the right) doesn't know that Jim (on the left) has installed a trick door on his side."
Jim hits the switch and the trick door on Fred's side pops open!
Which way does the box move?
With the door open and the back pressure released out that door, Jim's push to the left is what moves the box!"
"Think of the open door as the rocket nozzle. Fred's pushing effort is the escaping gas out the nozzle."
It's the upward push that gives the rocket it's movement.
This was one of those "AH HA!" moments.
My science teacher explained it in a way I could visualize and understand.