Originally, a novella by Daniel Keyes. Written in 1959 , it was awarded a Hugo award. It was later expanded into a novel in 1966, and won a Nebula for that incarnation. The 1968 movie Charly based upon it, and that spawned a 1980 Broadway musical. I think that's enough forms.

In the novella, Charlie Gordon, an friendly mentally retarded man, narrates his story in a series of "progris riports". He consents to an experimental procedure intended to increase his intelligence, and soon surpasses his fellow test partner, the mouse Algernon. Charlie's intelligence grows almost exponentially, and he experiences life as never before - until Algernon starts to sicken...

This is one of the saddest stories, but it's a valuable opportunity to think about Charlie's choice, and the reactions of those around him.