Hou Yi didn’t want to be immortal because he loved Chang Er too much and wanted to be by her side. No one knew if one got to stay on Earth once consuming the Elixir of Life so Hou Yi gave the pill to Chang Er for safekeeping instead. Hou Yi got really famous due to his accomplishment, and tons of people came knocking on his door to be his apprentice.
One of his apprentices, Feng Meng (逢蒙) was greedy and evil. And he wanted the elixir. One day, when Chang Er was home alone, Feng Meng demanded to have the Elixir of Life. Chang Er knew he was up to no good, so she did the only thing she could. She ate it.