Oobleck looks and behaves like a liquid, but you can walk over it as if you were walking on water.
Non-Newtonian fluids are usually liquid, but when force is applied they can change to be more solid or more liquid. Oobleck (a mixture of cornstarch and water) looks and behaves like a liquid, sloshing around like a pool full of water. However, if you step on it – applying force, in other words – it turns solid beneath your feet. So, if you’re quick and clever, you can walk on it and it looks a lot like you’re walking on water. You can see what this looks like in practice in the video below.
(Ketchup is also non-Newtonian, except that it gets runnier when you apply pressure. That’s why you have to hit the bottom of the bottle to get it out.)