I have been making this with children of all ages for over 6 years and I have yet to meet a child (or adult) who doesn't react to Oobleck by saying "coooooollll".
Age recommendation: Toddler, Preschool or early elementary.
Materials: Corn starch, water and a bowl.
How to play:
Full details and instructions can be found on the Scientific American website. Oobleck is an interesting substance to play with because it can be both a hard substance and a liquid substance at the same time. Use a small deep bowl for this activity. This way you can "punch" the substance and also place your entire hand in the bowl as well. This activity gets messy. Make sure you have your floors covered or do this outside. It is well worth the mess though!
Learning and Development: You can use this to develop scientific inquiry and also build on pincer grip. You can place one marble in a deeper bowl and make your Oobleck in this bowl. Ask your child to try to get the marble and take it out using only their thumb and index finger. You can also challenge them to make a ball with Oobleck. The faster you move the substance around in your palms, you will see a ball form. The moment you stop rolling your palms around it will liquify and drip back into the bowl. Ask questions about observation such as "how does it feel?" "what happens when you hit it quickly?" "how can you get your hand to sink to the bottom?".