Ya, its all explained in the books, Dr. Catherine Halsey started the Spartan MJOLNIR whatever project.
WARNING SPOILERS If you havent read the books yet you might not want to read the spoilers, but you can. Its your decision
Dr. Halsey came to earth with Captain Keyes(you know the captain in halo 1) and they searched. Halsey looked for a kid named John(master chief). It was a while ago when John was a kid. when Halsey found John she began speaking with him. Halsey told John that they were playing a game and John went like yay or something.So Halsey flipped a coin(a realy old quarter) and slamed it on her other hand hiding it) she asked which side did it fall on. John said the correct side. It all explained where like he saw the coin falling down and her slaming it on her hand. Kinda like saying he had realy good reflects or somethin. Then after that, John got to keep the quarter and ran off. He went playing with his friends.