This is what we did... but on a much smaller scale. =P

All you have to do is add cornstarch to water and stir it until it's the right consistency. It's pretty amazing... If you push on the top it's hard but if you leave your hand there it sinks in. We would take a little in our hands and roll it into a ball (because it would be hard) but the second that you stop applying pressure it turns into liquid and runs off your hand. It's SOOO much fun to play with! I couldn't find a video with English but this is ok haha! This looks a little more liquidy than what we made... but obviously it still works! =) Oh, and sorry, they're talking quite a bit of the time so you may want to skip some of it.