There are a lot of very strange challenges out there on the internet. This one isn't very strange, but it is very challenging. The Windmill Challenge is about health, flexibility, and martial arts performance.
Do NOT attempt this challenge unless you are sure of your stability and hip flexibility already. This can be used to improve hip flexibility and speed, but it requires a good amount of skill to even begin, so please use caution.
The challenge is to be able to do ten full circles without touching the ground with your swinging leg, and then repeat using the other leg.
Think you are up for the Windmill Challenge? Be safe, and if you can, give it a try.