Mind Blowing Science comes with cornstarch. Other than cornstarch, all you need to make "magic ooze," or a "crazy colloid," or "ooblek,"* or a "non-Newtonian fluid" is water. The kit instructions say to use 1/8 cup of water with 5 tablespoons of cornstarch, but we needed a bit more water for the consistency to be right. The more pressure you put on the ooze, the thicker it becomes. Poke it and it's solid, pour it and it's liquid. I've read that it's even possible to walk on cornstarch and water! (I haven't tried this, but it would be fun, wouldn't it?)
*Ooblek is named for the Dr. Seuss book Bartholomew and the Ooblek.