This is one you have to try. It's not robotics related at all, it just fun, easy and cheap. It will be messy.
A non-Newtonian fluid is a fluid that exhibits normal fluid properties under normal pressure, but when a force or pressure is applied, it acts like a solid. It is completely weird and will get your mind working, so you have to try it and feel it to completely get what I am talking about. I have about 2 cups of the stuff in my kitchen that I have been messing with for the last two days.