I love optical illusions and the fault lines they reveal in the human brain. This is a really good one. Stare hard at this image above, bearing in mind it may take a minute or so. You can open the image in a new link and enlarge it to stare at just one aspect to make it easier. What will happen is that your brain will start to dicsount it and it will disappear.
This is called the Troxler Effect. It is named for Swiss physician and polymath Ignaz Paul Vital Troxler who discovered the effect about 200 years ago. Scientists suggest this effect is created because the human brain is so good at adapting to new stimuli. When a stimuli becomes regarded as unimportant to the brain your neurons begin to disregard it is non-essential information and dismiss it. In this example, your brain will fade it to white, rending it invisible.
This same sort of mental muting happens to you every day, which is for the best. It allows your brain to dismiss irrelevant information like people talking around you, the sound of traffic or the bright shirt your co-worker is wearing.