Have a look at these photos and read the explanation.

Oobleck is a mixture of cornflour (custard powder works as well) and water named after a substance in a Dr. Seuss book. This liquid is a runny goo until you apply stress to it, and then it suddenly acts like a solid. You can hit a bowlful with a hammer, and instead of splashing everywhere, the particles lock together. You can roll it into a solid ball in your hand, but if you stop moving it, it reverts to liquid and oozes out through your fingers. In scientific terms, it is a non-Newtonian fluid (a fluid that changes its consistency under stress) like tomato sauce which becomes more liquid if you hit or shake the bottle. The reason why cornflour works so well to make oobleck is that it contains starch which does not dissolve in the water but instead is suspended in it. When the force applied, the particles of starch get mashed together, trapping water molecules between them, and the oobleck temporarily turns into a semi-solid material. A real-life example of this occurred during the 2010/2011 Canterbury earthquakes when the force of the quakes turned seemingly solid clay into a runny liquid (liquefaction). Our reason for doing this experiment is fun! Making oobleck is a great reason to make a mess, all in the name of science!