The Dancing Girl

Came across this really interesting gif today. It is claimed here that if you see the girl spinning anticlockwise, you use your left brain more than your right brain, and vice-versa if you see her spinning clockwise. That claim notwithstanding, I was quite intrigued to find out how exactly the animation can seem to move in both directions. A little bit of practice later, I was able to make her spin in either direction at will. Here's what you can try to control her direction in case you are unable to do so -
1. The simplest way would be to look away for a while and look back, and decide that you want to see her spin in a particular direction. That often works.
2. More specifically, decide that her left leg is in the air if you want to see her spin clockwise.
3. What worked best for me was looking at the shadow of her leg. If you focus on it continuously without looking at the entire image. you will notice that all it does is move from left to right, then right to left. In other words. if you tell yourself to concentrate on the left to right motion of the shadow and imagine her leg to be reversed during the right to left motion, you can control the direction of the dancer with great ease.

This led me to look up on a few things about how exactly it works, though point #3 above gives us a fair idea about what is happening. Since it's a silhouette, at no point can you tell whether the dancer is facing you, or facing away from you. That depends on your perception at that particular moment, and hence the direction of her motion also depends on your perception at that time.

Since I'm lukhha in life anyways, here's a few more links that were very interesting.
Here are a few comments about what people say about the illusion, and while some are quite insightful, most are just pretty stupid.
This is a paper recommended for understanding the phenomenon involved, albeit it went almost completely over my head. However the paper provides a compilation of various such perceptual rivalry phenomenon examples with some explanations here.
The animated necker cube, in particular, provides a visual and compelling explanation of illusions like these. Among the more astonishing illusions were Dale's Colour Cross (Do check the stepwise background removal), Dale Purves Illusion, Logveninko's Illusion, and the typical Checker Shadow Illusion that's been doing the rounds of internet recently.