If you mix corn flour (corn starch) and water together you get a really funky liquidy goop. This goop is often called oobleck and it has some really cool properties. This video is a fine example of oobleck in action:
The reason the guys in that video can walk on the liquid is because it is a non-Newtonian fluid. In this case, corn flour is made up of very long molecules (called starch), so if you hit the oobleck hard the molecules get caught up on one another and provide resistance kind of like a solid. But if you just slowly push on oobleck you will sink into it because the long molecules will have a chance to slowly slide past one another.
Another cool video explaining oobleck can be found here.
By the way, if you have never made oobleck before I strongly recommend it. Just go the supermarket and buy some corn flour, then mix it with water until the consistency is … well, you’ll know when you have it. Enjoy