The Grandma was given the coin by her Grandson while she was a little girl.
Many years passed and she gave the coin to her Grandson and told him to keep it.
More years pass and the Grandson invents a time machine.
(For the purpose of this answer I will assume a working time machine capable of going back prior to its own creation)
The Grandson goes back in time and gives the coin to his Grandmother and immediately returns to his present.
The coin, also known as an Artifact, came from an Ancient race that lives in the folds of time and are knows as the Illuminati - heard of them?
The Grandson encountered this race while traveling through time and spent many centuries learning from them some of the more mundane "secrets" of the Universe, such as "42" is not really the answer to life, the Universe and everything.
http://www.google.com/search?hl=en&q=a...+and+everything
He also learned the origins of Stonehenge, the statues on Easter Island, the drawings on the Nazca planes and all pyramids throughout the universe.
Sorry, but we do not have exclusive rights to pyramids on Earth.
He also learned how the "Great 'Wall' of China" was actually supposed to be a tower that the Chinese felt compelled to build so they could out do the Tower of Babel, but fell due to cheap foreign labor not being as skilled as the Hebrews, but hey, what are you going to do?
The Grandson showed such promise that the Illuminati offered to send him back to Earth's infancy and make him a lesser God.
The Grandson declined stating that he really needed to go back to Earth a few years prior to his invention of the time machine and make some key sports bets in order to finance his invention of said time machine. After all, if he did not travel back in time to finance his invention, he could never invent it!
But that's another issue in and of itself.
Besides, he loves knowing in advance that the Red Sox are going to win it all!
The Illuminiati are disappointed but understanding.
As a token of their mutual good will towards each other and the universe they gave him a year's supply of Rice-A-Roni and a lovely parting gift that came to be knows as "The Coin"
