"It’s a message. Clever boy left me a message."
-- Harry Potter (CC4.6)
Stuck in the past with Scorpius Malfoy when the Experimental Time Turner is destroyed, Albus Potter remembers that his father Harry has always spent time holding the blanket his mother covered him with on the anniversary of his parent’s murder by Lord Voldemort (CC1.7). Albus realises that, using his knowledge of potion ingredient reactions, he will be able to put a message on the blanket that will be read by Harry in the future (CC4.5, CC4.6).
He and Scorpius take Tincture of Demiguise from the unlocked house of Bathilda Bagshot. Albus uses it to write "Dad", the date (31 October 1981) and their location (Godric's Hollow), knowing that it will stay invisible for more than thirty years - until the blanket has love potion (an unwanted gift from his Uncle Ron) spilled over it (CC1.7, CC4.6).
Ginny has to help Harry decipher the writing on the blanket because his eyesight is so bad, even with glasses (CC4.6).
Pearl Dust, an ingredient in love potions, reacts with Tincture of Demiguise, but will stay invisible to the eye until the two ingredients come into contact with each other.
Before lighting upon this method of sending a message to the future, Albus and Scorpius try to come up with ideas on how they could manage it - including shouting at the baby Harry in his pram: "....it might traumatise the baby slightly" (CC4.5).
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About the play: http://www.harrypottertheplay.com