This may just be the easiest, messiest, and most fun science activity I know. It is a classic, and I have gotten several requests recently to post directions. You should know that if you try this activity and you are not smiling and messy with corn starch goo at the end, then you are definitely doing something wrong. Also keep in mind that this is not just about fun, there is some pretty amazing science going on here.
You will need:
Why does my Gloop act like that?
Your Gloop is made up of tiny, solid particles of cornstarch suspended in water. Chemists call this type of mixture a colloid.
As you found out when you experimented with your Gloop, this colloid behaves strangely. When you bang on it with a spoon or quickly squeeze a handful of Gloop, it freezes in place, acting like a solid. The harder you push, the thicker the Gloop becomes. But when you open your hand and let your Gloop ooze, it drips like a liquid. Try to stir the Gloop quickly with a finger, and it will resist your movement. Stir it slowly, and it will flow around your finger easily.
Smack water with a spoon and it splashes. Smack Gloop with a spoon and it acts like a solid.
Most liquids don't act like that. If you stir a cup of water with your finger, the water moves out of the way easily--and it doesn't matter whether you stir it quickly or slowly.
Your finger is applying what a physicist would call a sideways shearing force to the water. In response, the water shears, or moves out of the way. The behavior of Gloop relates to its viscosity, or resistance to flow. Water's viscosity doesn't change when you apply a shearing force--but the viscosity of your Ooze does.
Back in the 1700s, Isaac Newton identified the properties of an ideal liquid. Water and other liquids that have the properties that Newton identifies are call Newtonian fluids. Your Gloop doesn't act like Newton's ideal fluid. It's a non-Newtonian fluid.
There are many non-Newtonian fluids around. They don't all behave like your Gloop, but each one is weird in its own way. Ketchup, for example, is a non-Newtonian fluid. (The scientific term for this type of non-Newtonian fluid is thixotropic. That comes from the Greek words thixis, which means "the act of handling" and trope, meaning "change".)
Quicksand is a non-Newtonian fluid that acts more like your Gloop--it gets more viscous when you apply a shearing force. If you ever find yourself sinking in a pool of quicksand (or a vat of cornstarch and water), try swimming toward the shore very slowly. The slower you move, the less the quicksand or cornstarch will resist your movement.