Playing with oobleck

I was attracted by a plastic tray of whitish dough brought by my students. They were playing with it. They poked their fingers hard at the surface and there seemed to be a force counteracting them and their fingers did not go through. I tried to poke it with my forefinger and surprisingly, my finger seemed to meet an impenetrable surface. This phenomena really surprised me. Later, the students told me that it was called 'oobleck'.

Oobleck can be produced by fixing two parts of cornflour with one part of water. It seems that if you subject the tray of oobleck to strong vibration, it will part in the middle with a circular hollow in the middle. They tried it with their mobile phone vibrating but with no success. Then a student used her finger to move oobleck in a circular motion very fast and I could see the hollow that appeared in the middle.