what surprises me is that there are people high-fiving her as well? i'd be giving her a slap on the back of the head with a baseball bat. especially if it was my kid she had just done that to.
They're called cornstarch monsters. It's because a mix of cornstarch and water forms a non-Newtonian liquid. When pressure is applied, it becomes solid. Mythbusters, Brainiac, and Time warp all did items on it.