Bishop73

Answer: The book never included each of them giving the other the pocket watch. So it was added just for the film, but I don't know if the film makers did this to intentionally show audiences the paradoxes of time travel. This would be an example of a causality loop. The watch seems to exists solely because of time travel. This would be like if you were visited by your future self who tells you how to build a time machine and gives you the plans. You build the time machine and afterwards, go back in time to give your younger self the plans to build a time machine. So where did the plans come from? One solution basically states time was moving linear and at some point you invent time travel. Then once you go into the past, you created a loop. So at some point in the past, Elise had the pocket watch which she gives to Richard for some reason and when he gives it to her in the past, he creates the loop. But the film is just the loop, so we don't know where it comes from.

Bishop73