Disclaimer: I don't own Fullmetal Alchemist or Tangled's storyline.
The Philosopher's stone, a rare blood red stone that is believed to house the power to grant eternal youth and power beyond imagination. Many believed in it as a fable, a fairy tale meant to satisfy dreams. But when the King's wife fell ill during her last month of pregnancy, the king was willing to try anything, even search for the fabled stone.
The stone was quit real; in fact a man had been using it to keep himself young for a thousand years. It was located in a mysterious stone slab in the woods with a giant weird complex circle around it. The man could never free the stone from its prison in the slab.
When the search for the stone spread out into the woods, the guards stumbled upon the stone. The guard gently plucked the stone from its resting place. The man watched with anger for he himself could not move the stone.
The guards brought it to the King who ordered that it be made into an elixir and given to his wife. After she drank it, her illness disappeared and she bore 2 sons, twins.
The youngest one had sandy blonde hair with brown eyes, they named him Alphonse. The oldest had golden hair and a set of dazzling golden eyes, they named him Edward.
In the middle of the night, the man snuck into the castle to search for the stone. After not finding it he noticed the two young princes sleeping in the nursery. He walked over and took interest in Edward's golden hair; it was such an abnormal gold. As payback for stealing his youth, the man made off with the baby prince. The man took him to a tower in the middle of the woods and locked him away. He hid Edward from the world.
The man knew he had to hide the prince from discovery. He raised the boy in secrecy. At the age of 2 Edward began to develop abnormal powers. He was able to perform alchemy without the use of a transmutation circle. The man considered this a threat, and decided to handicap the young prince until he could train him correctly. At 8 years old he underwent the automail surgery. The man taught Edward science, math, alchemy, and even how to fight; Edward was smart and very studious. Every week the man would have Edward stand in the middle of transmutation circle and he would transmute Ed's philosopher's stone energy into life energy.
He kept the young prince safe, locked away from the world.