With very simple materials, students can create this strangely behaving non-Newtonian fluid. It acts like a liquid when handled slowly, but solidifies when handled quickly! (I usually use it in my middle school classroom to teach about atomic/molecular motion in solids and liquids).
A day or two day ago, I uploaded the lab to my Teachers Pay Teachers store.
(See it here - there's a free downloadable preview).
Due to some curriculum and grade level changes, I haven't had the opportunity to do this lab with my classes in a couple of years, but looking it over reminded me what a great learning experience it is - I've had kids come back to me years later and still remember it.
Below are some videos (not of me or my class) showing Oobleck's strange behavior.
Think you know how it works? The first three comments on this post get the lab and key for free! (Please leave your email address in the comment, or email it to me separately if you'd prefer).
Oobleck on a Speaker
Mythbusters Making (and Trying to Walk Across) Oobleck
There's also a recipe that involves glue and Borax.
I haven't used it, but I know others who have: