You know how often people say that science can be funny. This new video was posted by Physics Girl and it is really amazing. I think that the purpose of this was to explain something scientific on a very interesting way, and they did a good job, this could be a great lesson for children.
All you need to try something like this is a pool and a plate, and the magic can begin. When you try to drag a plate across the pool, you will see 2 black circles forming at the bottom and traveling through the pool (on a sunny day this is even more visible). Maybe you will think that this is something weird, but no. Apparently these 2 circles are called vortexes and if you watch and listen close the whole video you will hear the science explanation for this phenomenon.