The magical substance that falls from the sky in “Bartholomew and the Oobleck” is Oobleck.
In Dr. Seuss’s 1949 story, King Derwin becomes dissatisfied with ordinary weather and orders his Royal Magicians to create something new. They produce Oobleck at Mystic Mountain Neeka-tave and release it into the atmosphere, after which the strange green substance begins falling over the kingdom.
Oobleck is gelatinous and adhesive rather than liquid like rain. It gums up the royal bell, traps a robin, clogs instruments and sticks to people and objects. The king initially celebrates the unusual weather, but soon realizes that his attempt to control the sky has endangered everyone.
The crisis ends only when the king admits his mistake and says, “I’m Sorry.” The story’s Oobleck also inspired the name of a real classroom science material: a non-Newtonian fluid made from cornstarch and water. That experiment is related by name, though it is not literally the same substance as the magical goo in the book.