You’ve just made Oobleck, a non-Newtonian fluid. Most liquids are Newtonian fluids, staying either runny or thick at all times and changing from runny to thick only if the temperature changes. Non-Newtonian fluids can switch between acting runny and thick depending on how much force you use—just like your Oobleck.
Oobleck is made from cornstarch and water. Cornstarch has long molecules (or chains) of starch. When you add water, the long molecules dissolve in the water next to each other. If you move a spoon very slowly through the mixture, the fluid is runny and allows the long molecules to move aside and flow past each other. If you move a spoon quickly, the long molecules tangle up, the liquid becomes thicker, and movement stops.
This material gets its funny name from the Dr. Seuss book Bartholomew and the Oobleck. In the book, Oobleck is a
mystery substance that falls from the sky.