Matilda

Matilda Wormwood is a genius, but her parents, Harry and Zinnia (Danny DeVito and Rhea Perlman, respectively), ignore and mistreat her. At the age of 4, she discovers the local library and walks there every day to read while her father is at work, her mother is playing bingo, and her older brother, Michael (Brian Levinson), is at school.
At age six-and-a-half, Matilda begins to lose patience with her parents, expressing a desire to go to school. In retaliation for her father constantly berating her, she adds hydrogen peroxide to his hair tonic, turning his hair an unhealthy blonde. He takes his children to his workshop, where he reveals that the cars he sells are faulty. Matilda accuses him of being dishonest and he belittles her, so she responds by putting super glue on his hat, forcing Zinnia to cut it off of his head. She reads a borrowed library copy of Moby-Dick, which Harry rips up while her family is watching television. When he forces her to watch with them, she becomes increasingly angry and causes the television set to suddenly explode.
Agatha Trunchbull (Pam Ferris) is the tyrannical principal of a rundown elementary school, Crunchem Hall. Harry enrolls Matilda there, where she befriends several children and learns of Trunchbull's violent nature and overtly harsh punishments of the students. Matilda's teacher, Jennifer "Jen" Honey (Embeth Davidtz), is a kind woman who adores her class and takes an immediate liking to Matilda. She requests to Trunchbull that Matilda be moved up to a higher class, but Trunchbull refuses. That night, Miss Honey pays the Wormwoods a visit to encourage them to spend more time with Matilda, but they snub her. Meanwhile, Matilda discovers that her family is under surveillance by FBI agents Bob and Bill (Paul Reubens and Tracey Walter, respectively) due to her father's illegal dealings, but her parents refuse to believe her, as Zinnia thinks they are ace power speedboat salesmen.
Trunchbull goes to Miss Honey's class for a weekly "checkup" to belittle the students. As a prank, Lavender (Kiami Davael), one of Matilda's friends, places a newt in Trunchbull's water jug to frighten her. Upon discovery of the newt, Trunchbull accuses Matilda, whose anger at the injustice leads her to telekinetically tip the glass over, splashing water and the newt on Trunchbull. Afterward, Miss Honey invites Matilda to her house for tea. On the way, they pass Trunchbull's house, and Miss Honey reveals a secret of hers: When she was two, her mother died, so her father, Magnus, invited his stepsister-in-law, Trunchbull, to live with them and look after her. However, Trunchbull regularly abused her. When Miss Honey was five, her father died of an alleged suicide. Eventually, she moved into a small cottage. Matilda and she sneak into Trunchbull's house while she is out to obtain some of Miss Honey's belongings, but Trunchbull's unexpected return leads to a cat-and-mouse chase, with them only barely escaping without revealing themselves.
When Matilda's telekinetic powers manifest again during an argument with her father, she trains herself to use them at her own will. She returns to Trunchbull's house and uses her telekinesis to wreak havoc in an attempt to scare her away. She almost flees, but Trunchbull finds Matilda's hair ribbon and realizes that she was there. The following day, Trunchbull visits Miss Honey's class again to get Matilda to admit her guilt. Matilda uses her powers to write a message on the blackboard, posing as the ghost of Magnus, accusing Trunchbull of murdering him. Trunchbull goes berserk and attacks the students, but Matilda keeps them out of harm's way with her powers, and together they force Trunchbull out of the school permanently. Miss Honey subsequently moves back into her true home.
The FBI finally uncovers enough evidence to prosecute Harry, and the Wormwoods prepare to flee to Guam. They stop by Miss Honey's house to pick up Matilda, but she refuses to go with them and suggests Miss Honey adopt her. In that moment, a remorseful Zinnia laments not understanding her daughter better. She and Harry subsequently sign the adoption papers that Matilda had kept for a long time. They escape and Matilda lives a happy life with Miss Honey, who becomes the new principal of Crunchem Hall.

Question: What does Matilda add to her father's hair tonic?
Answer: hydrogen peroxide