"Okay," Severus said, patiently listening until Harry was done. "Let’s tackle this piece by piece. There was a lot he didn’t understand. "Firstly, Albus and Mr Flamel are storing a special stone in the castle for safekeeping. It is called the Philosopher’s Stone, one of the greatest by-products of Alchemy. Mr Flamel created it, and the stone produces an elixir that can grant the drinker the gift of immortality. No one should know it is here. It was previously kept safely in Gringotts, but Albus got it removed the very day of the break-in."