This 100 year old optical illusion recently created a stir on the internet.
It's known as an ambiguous image optical illusion, an image that can depict
two different pictures.
First published in 1892 in a German humor magazine it's the oldest known
depiction of an ambiguous optical illusion image.
Research by psychologist Joseph Jastrow showed that the faster
you can see the second animal, and how quickly you can switch between the
two, the quicker your brain works and the more creative you are.