It basically starts out all runny, but when you apply pressure to it, it turns into a hard solid ball.
Once pressure is released, it automatically turns into a runny, gloopy mess again. Apply pressure and it stars all over again.
If you want to make your own Oobleck, here is the recipe. I love it because it uses simple household items.
2 parts cornstarch
1 part water
food coloring of your choice
(FYI--the food coloring will stain your hand, but it should come off with soap and water)
Mix all items together and play until your heart's content!