When I taught second grade, we did this activity when we learned about matter. When you make Ooblek, you can pick up chunks of it and put it in your hand and then it will "melt" and drip between your fingers. Is it a solid? Is it a liquid?
I let my kids explore with craft sticks at first and then they were loving getting their hands in it. One of our favorite things to do was put our whole hand, palm down, in the middle of the pie plate and have the Ooblek suck your hand down and then it was hard to get your hand back out.
I threw some buttons from our last snowman craft (can you believe that I hadn't cleaned up yet ;) ) in the Ooblek, pushed them down so they were covered, and had them have a treasure hunt. They had a ton of fun.
1 cup corn starch
2/3 cup water
you can add food coloring in as well if you want
This is a VERY MESSY activity. My suggestion is to do it right before you plan on cleaning anyways HAHA!