Black tells them he meant to kill Peter Pettigrew twelve years ago but did not manage to do it. He tries to take Scabbers from Ron. Lupin stops him, telling him they need to explain everything first since Harry, Ron and Hermione have the right to know everything. Peter Pettigrew is an unregistered Animagus, one of three which used to be students together at Hogwarts. Lupin was bitten as a child by a Werewolf. The potion which Professor Snape has been making for him, the Wolfsbane Potion, keeps Lupin's mind stable when he is transformed so that he can simply wait for it to pass. Dumbledore knew about Lupin's state. He had the Whomping Willow planted the year Lupin attended Hogwarts. The Shrieking Shack was built as a place for protection when he was transformed. Black, Pettigrew and James Potter, his best friends, found out and became Animagi to keep him company during his time as a Werewolf. They created the Marauder's Map. Sirius' knickname was Padfoot, Peter's was Wormtail, James' was Prongs and Lupin's nickname is Moony. Because of his gratitude toward Dumbledore, Lupin never had the courage to tell him that Sirius is an animagus.
When they were students on Hogwarts, Sirius played a trick on Snape. Since Snape was very curious about the four's whereabouts, Sirius told Snape how to enter the Whomping Willow when Lupin was being led to the tunnel by Madam Pomfrey. To save Snape from meeting with a fully grown Werewolf, James Potter pulled snape back at great risk to his own life. Snape however saw Lupin as a Werewolf but was forbidden to talk about it by Dumbledore. At that moment Snape appears from under Harry's invisibility cloak.