Enemy
When Sam was five, his biggest enemy was the monster who lived under his bed. He was big with sharp teeth and Sam called him Dennis.
When Sam was ten, and fifteen, and sixteen if he was honest, his biggest enemies were the bullies who tormented him at school and the teachers who never took his side.
When Sam was seventeen, his enemies took the form of giant robots, designation Decepticons, who wanted to destroy his world. Did he mention they were alien?
It really was funny how time could change things.