One thing I love to do with the girls is mix up corn starch and water. It is thick and goopy, and at the right mixture, it turns into a solid when force is applied. So when you scoop up some in your hand and squeeze it it turns from runny into this solid play-doh like stuf, and tehn shifts back into ooze.

Here is a video of corn starch that is being bounced around by someone’s speaker:

You can also find it here.