"I put in another cube and sent it two years back. The second cube came back unchanged, except that it was newer, shinier.
"That gave me the answer. I had been expecting the cubes to go back in time, and they had done so, but not in the sense I had expected them to. Those metal cubes had been fabricated about three years previously. I had sent the first one back years before it had existed in its fabricated form. Ten years ago it had been ore. The machine returned it to that state.
"Do you see how our previous theories of time travel have been wrong? We expected to be able to step into a time machine in, say, 2004, set it for fifty years back, and then step out in the year 1954 ... but it does not work that way. The machine does not move in time. Only whatever is within the machine is affected, and then just with relation to itself and not to the rest of the Universe.
"I confirmed this with guinea pigs by sending one six weeks old five weeks back and it came out a baby.
"I need not outline all my experiments here. You will find a record of them in the desk and you can study it later.
"Do you understand now what has happened to you, Norman?"
You begin to understand. And you begin to sweat.
The I who wrote that letter you are now reading is you, yourself at the age of seventy-five, in this year of 2004. You are that seventy-five-year-old man, with your body returned to what it had been fifty years ago, with all the memories of fifty years of living wiped out.
You invented the time machine.