Poetry has existed since before the development of writing and literacy. Ancient poetry fell into three main categories: epic, comic, and tragic. As literacy grew, poetry became more personal and lyrical in form. Homer's epic poems the Iliad and Odyssey had huge influence and helped establish common story themes still used today. Poetry has often been used as a medium for revolution or expressing dissent, such as by American colonists protesting British rule or slaves sharing songs of their hardships. It can also act as a subversive art form that promotes new ideas before their time.