Well, water and cornstarch. This is awesome, I really want to get some cornstarch and mess with it. When mixed with water, a non-newtonian fluid is made. This is a type of fluid without a specific viscosity. When pressure is applied to the liquid, the molecules stiffen in a fixed point, giving the properties of a solid. This allows these guys to walk across it. Fucking cool, science is awesome
http://www.youtube.com/watch?v=f2XQ97XHjVw