Non-newtonian liquid

Newton described how normal (Newtonian) liquids or fluids behave, and he observed that they have a constant viscosity. This means that their flow behaviour or viscosity only changes with changes in temperature or pressure. Non-Newtonian fluids change their viscosity or flow behaviour under stress. The sudden application of stress (sound waves in this case) can cause fluid to get thicker and act like a solid, or in some cases it results in the opposite behaviour and they may get runnier than they were before. Remove the force and they will return to their earlier state.

In this case we have a mixture of potato starch and water which act like a liquid when you stir it slowly, but when the force is applied liquid starts act like a solid. Other examples of non-Newtonian fluids include ketchup, silly putty, corn starch and quicksand. For example if you struggle to escape quicksand, you apply pressure to it and it becomes hard, making it more difficult to escape. Ketchup is the opposite, its viscosity decreases under pressure. That is why shaking a bottle of ketchup makes it easier to pour.