In undergrad we learned about non-Newtonian fluids. Normally fluids that we’re familiar with (like water) have a constant viscosity. However, some fluids change their viscosity depending on strain rate. That is: they get thicker or thinner when you move something through them.
The quintessential example of this, often used in demonstrations, is to mix cornstarch and water in the right proportions. Then, if you put your finger in the fluid slowly, it goes right in. However, if you rapidly poke the surface of the fluid, it firms up, your finger is stopped, and leaves a dent in the surface, and if you take your finger away as rapidly as you inserted it, you typically end up with nothing on your finger.
I always wondered if it would be possible to run across a big pool of this stuff, provided one kept moving. Well, thanks to the Internets, I wonder no more. Observe:
Thanks to babayada for the initial link to YouTube (to another cornstarch and water video) via which I found this.
ADDENDUM: I just noticed that the very Wikipedia article I linked also references this video. Hah!