science nerd has an explanation for the first one with ellen...
the cornstarch-water mixture is classified as a Non-Newtonian fluid
. The molecules are shaped like L's. When you hit them hard, the lock together. But, when you only touch them softly, they slide past one another, avoiding the locking that makes them a solid.
Here's another video with more tricks you can do with this.
If you put it on a speaker and emit some sort of frequency, you get this effect where pseudopod-esque things come out at you and dance around. Also, if you were to blow a hole to the bottom, it stays there much unlike a regular fluid.
is another game show where they filled a tub with this stuff..