Beatrice is the central figure around whom the play revolves. She is a single mother who was left by her husband years ago. This event has fostered a deep distrust of men. She still lives in the same house in which she grew up and has become increasingly reclusive over the years. Beatrice still mourns the loss of her father, a man whom she was forced to confine to a sanatorium years ago. The world has caused her a lot of pain, and she takes it out on those around her. She vents her hostility primarily upon her two teenage daughters. Beatrice has always yearned to be popular, but she has always been an outcast. She is desperate to escape her circumstances and constantly dreams of a better life.
Ruth is Beatrice's older daughter. She is an epileptic whose seizures are brought on by anxiety or stress. Ruth is somewhat promiscuous and is very concerned about her appearance. She constantly worries about Tillie, her younger sister, embarrassing her at school. Ruth is very fickle in her relationship to Tillie. She often makes fun of her, but as soon as Tillie wins the science fair and becomes somewhat of a school celebrity, Ruth is very quick to brag, ‘‘That's my sister.’’ Ruth has a quick temper and is not afraid to talk back to her mother. Her attempts to lash out and hurt her mother are what drive much of the action of the play.
Tillie is Beatrice's younger daughter. She is an outcast at school and is teased by the other students. Tillie is very intelligent, and her teacher, Mr. Goodman, encourages her interest in science. The title of the play comes from the experiment Tillie enters in the school science fair. Tillie is a dreamer who yearns for a better world. She is quiet and thoughtful. She is also somewhat awkward and is often chastised by her mother for this.
Nanny is the boarder who lives in the spare room. She is nearly blind, deaf, and can barely walk with the aid of a walker. Nanny's daughter has given up responsibility for her care.
Janice is Tillie's main competitor at the science fair. She gives a gruesome but funny presentation in which she describes boiling a cat and collecting the bones to reconstruct the skeleton. Janice's speech provides comic relief right before the dramatic climax of the play.