Oobleck

2 parts cornstarch to 1 part water
food coloring as desired


This is a wonderfully fun substance to play with. It acts like a solid when you add pressure and a liquid when you don't, but don't tell them that; let them discover it on their own! Ask them "When does this seem like a liquid? When does it seem like a solid?"
Older students may be interested in exploring Non-Newtonian fluids as a way of describing Ooblek. Adding the cornstarch to the water changed the reaction time of the liquid. Usually liquids move so quickly we do not notice how long it takes them to move. Oobleck has a finite reaction time. We can see how long it takes for it to react. We can make oobleck act like both a solid and a liquid. A liquid like this is called a non-Newtonian fluid because it does not follow the rules that Newton discovered most liquids follow. A non-Newtonian fluid has properties of both a solid and a liquid and reacts to stress with increased viscosity. Cornstarch (amylose) and water can be considered a colloidal suspension. A colloidal suspension is a two-phase system in which the starch and water are not dissolved but simply mixed into a permanent suspension that will not settle on standing.
For younger kids you can read them Bartholomew and the Ooblek by Dr. Seuss
You can let this dry out and put it in a baggie to reconstitute again later.