Short Short: The Little Box Magic

A small box was found inside the sandbox at the park. It was small enough to fit in the palm of my hand, and when I opened the lid, it was empty. I put a pebble from nearby inside and brought it home, listening to it rattle. When I got home, I saw my school bag tossed in the living room. Without thinking, I just put the box inside the school bag.
The next morning, when I went to switch out the contents of my school bag, the box came out. I had completely forgotten that I put it in there yesterday. When I opened the lid, there were two pebbles inside. Moreover, they were pebbles of the exact same color and shape. I thought it was strange, but since I didn't need the pebbles, I threw them out the window, put my textbooks and the box into my school bag, and hurried off to school.
In the middle of the first period, the kid sitting next to me said, "I forgot my eraser, can I borrow yours?" When I reached for an eraser in my pencil case, I suddenly remembered the box and took it out of my school bag.
I said, "I'll show you a magic trick now," tore the eraser in half, put it in the box, shook it two or three times, and then showed them the inside of the box. Then, the torn eraser had become two. I handed one to the kid next to me with a smug look on my face, but they didn't seem very surprised, just said "Thanks," and started taking the class as if nothing had happened. Feeling dissatisfied that they could have been a little more surprised, I also returned to the lesson.
After I got home, I showed the magic trick to my family. At first, they were happy, saying "Wow, that's amazing!" but they immediately started asking about the secret or the trick behind it. I replied, "There's no such thing, you just put it in this box and shake it, and it becomes two! Isn't it amazing!"
Then, my father looked at the box intently, and after having an epiphany, he said excitedly, "Let me borrow this box!" When I handed him the box, he took a 10,000 yen bill out of his wallet, crumpled it up, and shoved it into the box. After shaking the box, he took out the crumpled 10,000 yen bill and unfolded it, and it had become two. My father was overjoyed and showed me the two bills. When I said, "Dad, that's amazing! The shiny stickers and the tiny numbers are all exactly the same!" my father's face clouded over.