This was always a hit with my class and a great way to mix literature and science in one lesson. Start off by reading Bartholomew and the Oobleck
Discuss with your kids how they think you should make Oobleck. Put out ingredients and let them experiment with different mixtures. If the weather is nice head outside to do this and you won't have to worry about the mess. Then after they have come up with their own mixture whip up some Oobleck and let them compare the results.
1 part cornstarch
1 part water
Food coloring (could tint hands)
Mix all the ingredients together.
Some fun things to observe about OOBLECK:
*Oobleck is a solid and a liquid at the same time.
*If you make a ball and throw it up in the air, it loses shape and will flatten in the air.
*Oobleck will mold to the shape of a container but when taken out it will flatten.
*If you fill a balloon with 0obleck, the air pressure will force the Oobleck back out.
*When you leave Oobleck in the sun, the color evaporates, it turns hard like cement on the top and like jelly on the bottom. When heated this also happens and it is sticky.
Picture Credit to Science Cafe