"Okay, one last analogy, then we have to move on whether you get it or not.  Consider this scenario:

On Saturday afternoon, I park in my personal parking space and a tree falls onto my car.  I go back in time to Friday and tell myself to park my car on the street.

But on Saturday morning I get a ticket for being in a street cleaning zone and my car is towed.  So I go back to Thursday and tell myself to park in the street, but on the other side where it will be legal to park."

I interrupted,  "Why don't you just go back to Friday again and tell either one of you to do it right?"

Other me points at real me and smiles:  "Good catch.  I forgot that part.  You can't go back to the same day twice."

"Why not?

"I don't know, it just doesn't work."

"But that doesn't make any sense without a reason.  You should..."

Old me was becoming impatient.  "Of course you should be able to, and maybe one day we will be able to.  Time travel is still in its infancy.  Once upon a time we could transmit audio signals through the air but not video.  Video was finally figured out with TV, but you couldn't do it in color.  I'm sure we'll be able to make repeat trips in time one day, but likely not in my - our - lifetime.  You've got to play with the cards that you're dealt."

His analogy was reasonable, but I wanted to continue debating, "But..."

"No more questions now, you're throwing me off track.  Let me finish:

In the newest timeline, on Thursday I arrive and tell myself where to park the car.  Friday comes, and the version of me that made that original trip still makes it, but sees the car in the safe spot and doesn't do anything about it."

This time I actually understood:  "Which is why you didn't know that you had already injected me, because you haven't yet."

"Exactly.  So the one we'll call the 'youngest' me will move the car when told to on Thursday.  He'll see the tree fall on his parking spot, realize that his other self was right.  In order to ensure his car isn't ruined, he now has to go back in time to Thursday and tell himself to move it.  Now we've erased two incidents:  the tree actually falling on the car, and the botched trip to Friday.  Nobody will ever remember them, so it's as if they never happened."

"Wow.  This is deep, but very cool.  So what happens if you don't make that trip in the 'good' timeline?"

"Nobody knows, but nobody wants to risk it.  Don't want to fuck up this gift by possibly unravelling everything, do you?  It's a theory, but the risk is too great."