My non-Newtonian fluid

What is needed

• Cornstarch (available at the grocery store)
• Water
• Bowl or dish

That's it!

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What to Do

This is a very messy experiment - but it cleans up really easily. Go somewhere that you can make a bit of a mess, and do this:

• Take some cornstarch, and put some in the bowl.
• Add water, slowly. As you add the water, stir the water into the cornstarch. Don't use a spoon - use your hands to do this. This is part of the fun, and you will also be able to tell when you have put in enough water!

The key is to add just enough water so that the cornstarch and water mix will flow very, very slowly. Now you have something which is a very strange goo indeed.
Here are some things to try:

• Smack it! If you hit the mixture really hard, it might even break.
• Pour it from hand to hand. If you go slowly, it will pour freely.
• Roll it into a ball. If you keep rolling, you can roll it into a nice ball. Of course, as soon as you stop rolling, it will ooze and turn back into a puddle.
• Bounce it! If you get the right consistency, you can even bounce a ball of this goo!

What is Happening
The cornstarch is made up of long chains of atoms - a polymer. These chains can move past each other, but they take some time to do this. If you pour it slowly, it can flow like a liquid. If you try to force things and make the chains slide more quickly than they want to, they get entangled - and the mixture gets firmer. So if you push hard on it, it acts more like a solid.
The real explanation is actually a bit more complicated than this. But the exact workings of mixtures like this is something that is not all that well understood. Folks sometimes get upset when there is no really good answer for things in science, but really that's a good thing - if we already knew everything there was to know, there would be no reason to try new things - and what's the fun in that?
Other Things to Try
If you add different amounts of water, you can get mixtures that work very differently. You can also try different liquids. Corn syrup and cornstarch can make a mixture that will do weird things in the presence of static electricity, for instance.