After leaving Hogwarts, Snape became a Death Eater and informed Voldemort of half of an overheard prophecy, which Voldemort takes to refer to Lily and her son, Harry. Fearing for Lily's life, Snape joins the Order as a spy for Dumbledore, in exchange for what he hopes will be Dumbledore's protection of Lily. Voldemort offered Lily the chance to step aside before he killed Harry because of Snape's request to spare Lily's life, but Lily refused and Voldemort killed her. The result of her selfless act of love manifested itself two ways: when Voldemort attempted to kill Harry with the Killing Curse the spell backfired, rendering Voldemort non-corporeal. The lingering protection afforded to Harry by Lily's sacrifice rendered Voldemort unable to touch him physically. The second way in which Harry is protected by Lily's sacrifice occurs when Petunia takes Harry in. Dumbledore told Harry that he had extended Lily's protection to Privet Drive because Lily and Petunia are related by blood. This protection ends when Harry comes of age at 17.