Here is a fun and easy (and a little messy) activity that gives your kids a chance to play with some science.
I called this “Ooblek” when I made it as a kid, but I’ve heard is called slime or goo and lots of other names. All you’re really doing here is mixing cornstarch and water. Part of the fun is playing around with different ratios of water and cornstarch to see what happens, but you’re aiming for something like 1 part water to 1.5 or 2 parts cornstarch. (1/2 cup of water with 3/4 or 1 cup cornstarch, for example.) If you want to add a color, the easiest way is to add food coloring to the water before you start mixing it together.
As you are mixing the water and cornstarch together, you’ll start to notice that this stuff has some funny properties. The more you stir it or push on it with your hands the harder it gets. You can move your hand slowly through it, but it gets stuck when you try to go fast. If you squeeze some in your hand it will harden, but let go and it will start to drip out over your hand. It’s like it switches back and forth between feeling like a solid and feeling like a liquid.
Here’s why: The viscosity of this stuff changes related to force, which means that it doesn’t follow Newton’s Law of Viscosity. This makes it an example of a Non-Newtonian fluid. Viscosity is basically how “thick” a liquid is. Water has low viscosity, while something like honey has higher viscosity. Newtonian fluids (like water and honey) follow a rule where the viscosity only changes if you change the temperature. Have you ever heated up a bottle of honey because it was so thick that it was stuck in the bottle? It should flow out of the bottle more easily because it has become less viscous at the higher temperatures. The temperature change led to a viscosity change. Non-Newtonian fluids don’t act like this. They can change viscosity without changing temperature. This one changes related to stress or force. When you push on it (add force), it gets more viscous. When you release the force, it gets less viscous.
Give it a try. You’ll be learning about viscosity and Non-Newtonian fluids. Plus, it’s really fun!