Here is a really great thing to try at home (with or without children).
You will need:
a bowlWhat to do:
cornflour or custard powder (but not instant custard)
an egg cup
1. Put 8 egg cups of cornflour or custard powder into the bowl.
2. Add 3 egg cups of water.
3. Stir gently with the spoon, adding more water if it's too dry.
4. Stir it quickly and see what happens!
5. See if you can shape it into a ball and bounce it on the countertop! What you have made is called a dilatant liquid. You should find that as long as you handle it quite roughly, it will behave like a solid, but when you are too gentle, it turns back into a liquid and drips through your fingers!
If you make enough of the stuff and have somewhere to put it, you can actually run across the top of it. Be warned - if you stand still, you'll sink, so don't make it too deep! See here for Jonathan Sanderson's article 'Walking on custard' Jonathan's Web Pages can be found here
The Science Bit
Cornflour and water combine to make a non Newtonian fluid. See here for the physical properties of chemicals. The viscocity (thickness) of non Newtonian fluids is affected by shear forces (stirring) as well as by temperature. This leads to some pretty odd behaviour. Dilatant fluids get thicker the more you stir them, whilst thixotropic fluids get thinner. Examples of thixotropic fluids are ketchup, and non-drip paints. These become runnier the more you stir them which is why ketchup comes out of the bottle more easily if you shake it. When you stop stirring or shaking these liquids, they thicken again.
And here is a video I found showing how you can 'Walk on Water'!
Notice: I cannot find the creators of this video to ask permission. If it is yours and you would prefer I did not use it, or would like me to put your name to it, please email me. Thanks.
Noticia: No puedo encontrar la gente quien han hecho este video por conseguir su permiso. Si esto es lo suyo, y quiere usted que no lo utilizo aqui, o si quieren que su nombre puesto, por favor, mandame un email. Gracias.