Ooblick (Magic Mud)
Materials: corn starch, water, flat pan (pie plates or dish basins work well), newspaper, hammer
Directions:
You've created magic mud! This material is not quite a liquid, and not quite a solid, but somewhere in between. When pressure is applied to the mud, it behaves like a solid. When the pressure is released, the mud acts like a liquid. Basically what's happening is the long molecules that make up the corn starch get easily tangled together when under pressure, and are able to slide by each other when there is no pressure applied.
Note: this is a very messy experiment but lots of fun! If at all possible, do this outside. Makes the cleanup easier!