Sink Your Fingers Into This...

Cornstarch and water make a super fun, simple goo.  It acts as a solid and as a liquid!  It's a bit messy, but easy to clean and doesn't stain (unless you add food coloring).  My kids were entertained and engaged for a good 45 minutes and keep asking to make this again and again. 

(For more info on the science behind this non-Newtonian fluid you can check out this website.)